Steel sheet, coated steel sheet, and methods for manufacturing same

ABSTRACT

A steel sheet having TS of 590 MPa or more and YR of 68% or more is obtained by providing a predetermined chemical composition and a predetermined steel microstructure, where an average aspect ratio of crystal grains of each phase (polygonal ferrite, martensite, and retained austenite) is 2.0 or more and 15.0 or less, wherein the polygonal ferrite has an average grain size of 6 μm or less, the martensite has an average grain size of 3 μm or less, the retained austenite has an average grain size of 3 μm or less, and a value obtained by dividing a Mn content in the retained austenite in mass % by a Mn content in the polygonal ferrite in mass % equals 2.0 or more.

TECHNICAL FIELD

This disclosure relates to a steel sheet, a hot-dip galvanized steelsheet, a hot-dip aluminum-coated steel sheet, and an electrogalvanizedsteel sheet, and methods for manufacturing the same, and in particularto a steel sheet with excellent formability and hole expansionformability and high yield ratio that is preferably used in parts in theindustrial fields of automobiles, electronics, and the like.

BACKGROUND

In recent years, enhancement of fuel efficiency of automobiles hasbecome an important issue from the viewpoint of global environmentprotection. Consequently, there is an active movement to reduce thethickness of automotive body components through increases in strength ofsteel sheets as automotive body materials, and thereby reduce the weightof automotive body itself.

In general, however, strengthening of steel sheets leads todeterioration in formability, causing the problem of cracking duringforming. It is thus not simple to reduce the thickness of steel sheets.Therefore, it is desirable to develop materials with increased strengthand good formability. In addition to good formability, steel sheets witha tensile strength (TS) of 590 MPa or more are required to have, inparticular, enhanced impact energy absorption properties. To enhanceimpact energy absorption properties, it is effective to increase yieldratio (YR). The reason is that a higher yield ratio enables the steelsheet to absorb impact energy more effectively with less deformation.

Moreover, in the case of using a steel sheet in an automotive body,stretch flanging according to the shape of the automotive body isperformed, so that excellent hole expansion formability is required,too.

For example, JPS61157625A (PTL 1) proposes a high-strength steel sheetwith extremely high ductility having a tensile strength of 1000 MPa orhigher and a total elongation (EL) of 30% or more, utilizing deformationinduced transformation of retained austenite.

In addition, JPH1259120A (PTL 2) proposes a high-strength steel sheetwith well-balanced strength and ductility that is obtained from high-Mnsteel through heat treatment in a ferrite-austenite dual phase region.

Moreover, JP2003138345A (PTL 3) proposes a high-strength steel sheetwith improved local ductility that is obtained from high-Mn steelthrough hot rolling to have a microstructure containing bainite andmartensite after subjection to the hot rolling, followed by annealingand tempering to cause fine retained austenite, and subsequentlytempered bainite or tempered martensite in the microstructure.

CITATION LIST Patent Literature

PTL 1:JPS61157625A

PTL 2: JPH1259120A

PTL 3: JP2003138345A

SUMMARY Technical Problem

The steel sheet described in PTL 1 is manufactured by austenitizing asteel sheet containing C, Si, and Mn as basic components, and subjectingthe steel sheet to a so-called austempering process whereby the steelsheet is quenched to and held isothermally in a bainite transformationtemperature range. During the austempering process, C concentrates inaustenite to form retained austenite.

However, while a high concentration of C beyond 0.3% is required for theformation of a large amount of retained austenite, such a high Cconcentration above 0.3% leads to a significant decrease in spotweldability, which may not be suitable for practical use in steel sheetsfor automobiles. Additionally, the main objective of PTL 1 is improvingthe ductility of steel sheets, without any consideration for the holeexpansion formability, bendability, or yield ratio.

PTLs 2 and 3 describe techniques for improving the ductility of steelsheets from the perspective of formability, but do not consider thebendability, yield ratio, or hole expansion formability of the steelsheet.

To address these issues, it could thus be helpful to provide a steelsheet, a hot-dip galvanized steel sheet, a hot-dip aluminum-coated steelsheet, and an electrogalvanized steel sheet that are excellent informability and hole expansion formability with TS of 590 MPa or moreand YR of 68% or more, and methods for manufacturing the same.

Solution to Problem

To manufacture a high-strength steel sheet that can solve the aboveissues, with excellent formability and hole expansion formability aswell as high yield ratio and high tensile strength, we made intensivestudies from the perspectives of the chemical compositions andmanufacturing methods of steel sheets. As a result, we discovered that ahigh-strength steel sheet with high yield ratio that is excellent informability such as ductility and hole expansion formability can bemanufactured by appropriately controlling the chemical composition andmicrostructure of steel.

Specifically, a steel sheet that has a steel composition containing Mn:2.60 mass % or more and 4.20 mass % or less, with the addition amountsof other alloying elements such as Ti being adjusted appropriately, ishot rolled to obtain a hot-rolled sheet. The hot-rolled sheet is thensubjected to pickling to remove scales, retained in a temperature rangeof [Ac₁ transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 s, and optionally cold rolledat a rolling reduction of less than 30% to obtain a cold-rolled sheet.Further, the hot-rolled sheet as annealed after the hot rolling or thecold-rolled sheet is retained in a temperature range of [Ac₁transformation temperature+10° C.] to [Ac₁ transformationtemperature+100° C.] for 20 s to 900 s, and subsequently cooled.

Through this process, the hot-rolled sheet or the cold-rolled sheet hasa microstructure that contains, in area ratio, 20% or more and 65% orless of polygonal ferrite, 8% or more of non-recrystallized ferrite, and5% or more and 25% or less of martensite, and, in volume fraction, 8% ormore of retained austenite, where the average aspect ratio of crystalgrains of each phase (polygonal ferrite, martensite, and retainedaustenite) is 2.0 or more and 15.0 or less, the polygonal ferrite has anaverage grain size of 6 μm or less, the martensite has an average grainsize of 3 μm or less, and the retained austenite has an average grainsize of 3 μm or less. Moreover, the microstructure of the hot-rolledsheet or the cold-rolled sheet can be controlled so that a valueobtained by dividing a Mn content in the retained austenite (in mass %)by a Mn content in the polygonal ferrite (in mass %) equals 2.0 or more,making it possible to obtain 8% or more of retained austenite stabilizedwith Mn.

This disclosure has been made based on these discoveries.

Specifically, the primary features of this disclosure are as describedbelow.

1. A steel sheet comprising: a chemical composition containing(consisting of), in mass %, C: 0.030% or more and 0.250% or less, Si:0.01% or more and 3.00% or less, Mn: 2.60% or more and 4.20% or less, P:0.001% or more and 0.100% or less, S: 0.0200% or less, N: 0.0100% orless, and Ti: 0.005% or more and 0.200% or less, and optionally furthercontaining, in mass %, at least one selected from the group consistingof Al: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, with thebalance consisiting of Fe and inevitable impurities; and a steelmicrostructure that contains, in area ratio, 20% or more and 65% or lessof polygonal ferrite, 8% or more of non-recrystallized ferrite, and 5%or more and 25% or less of martensite, and that contains, in volumefraction, 8% or more of retained austenite, where an average aspectratio of crystal grains of each of the polygonal ferrite, themartensite, and the retained austenite is 2.0 or more and 15.0 or less,wherein the polygonal ferrite has an average grain size of 6 μm or less,the martensite has an average grain size of 3 μm or less, the retainedaustenite has an average grain size of 3 μm or less, and a valueobtained by dividing a Mn content in the retained austenite in mass % bya Mn content in the polygonal ferrite in mass % equals 2.0 or more.

2. The steel sheet according to 1., wherein the retained austenite has aC content that satisfies the following formula in relation to the Mncontent in the retained austenite:

0.09*[Mn]−0.026−0.150≤[C]≤0.09*[Mn]−0.026+0.150

where

[C] is the C content in the retained austenite in mass %, and

[Mn] is the Mn content in the retained austenite in mass %.

3. A coated steel sheet comprising: the steel sheet according to 1. or2.; and one selected from a hot-dip galvanized layer, a galvannealedlayer, a hot-dip aluminum-coated layer, and an electrogalvanized layer.

4. A method for manufacturing a steel sheet, the method comprising:heating a steel slab having the chemical composition according to 1.;hot rolling the steel slab with a finisher delivery temperature of 750°C. or higher and 1000° C. or lower to obtain a steel sheet; coiling thesteel sheet at 300° C. or higher and 750° C. or lower; then subjectingthe steel sheet to pickling to remove scales; retaining the steel sheetin a temperature range of [Ac₁ transformation temperature+20° C.] to[Ac₁ transformation temperature+120° C.] for 600 s to 21,600 s;optionally cold rolling the steel sheet at a rolling reduction of lessthan 30%; and then retaining the steel sheet in a temperature range of[Ac₁ transformation temperature+10° C.] to [Ac₁ transformationtemperature+100° C.] for 20 s to 900 s and subsequently cooling thesteel sheet, to manufacture the steel sheet according to 1. or 2.

5. The method according to 4., wherein a value obtained by dividing avolume fraction of the retained austenite after performing tensileworking with an elongation value of 10% by a volume fraction of theretained austenite before the tensile working equals 0.3 or more.

6. The method according to 4., comprising after the cooling, eithersubjecting the steel sheet to one selected from hot-dip galvanizingtreatment, hot-dip aluminum coating treatment, and electrogalvanizingtreatment, or subjecting the steel sheet to hot-dip galvanizingtreatment and then to alloying treatment at 450° C. or higher and 600°C. or lower, to manufacture the coated steel sheet according to 3.

Advantageous Effect

According to the disclosure, it becomes possible to provide ahigh-strength steel sheet with excellent formability and hole expansionformability and high yield ratio that exhibits TS of 590 MPa or more andYR of 68% or more. Steel sheets according to the disclosure are highlybeneficial in industrial terms, because they can improve fuel efficiencywhen applied to, for example, automobile structural parts, by areduction in the weight of automotive bodies.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawings:

FIG. 1 illustrates the relationship between the working ratio of tensileworking and the volume fraction of retained austenite; and

FIG. 2 illustrates the relationship between the elongation of each steelsheet and the value obtained by dividing the volume fraction of retainedaustenite remaining in the steel sheet after subjection to tensileworking with an elongation value of 10% by the volume fraction ofretained austenite before the tensile working.

DETAILED DESCRIPTION

The following describes the present disclosure in detail.

First, the reasons for limiting the chemical composition of the steel tothe aforementioned ranges in the present disclosure are explained. The %representations below indicating the chemical composition of the steelor steel slab are in mass % unless stated otherwise. The balance of thechemical composition of the steel or steel slab is Fe and inevitableimpurities.

C: 0.030% or More and 0.250% or Less

C is an element necessary for causing a low-temperature transformationphase such as martensite to increase strength. C is also a usefulelement for increasing the stability of retained austenite and theductility of steel. If the C content is less than 0.030%, it isdifficult to ensure a desired area ratio of martensite, and desiredstrength is not obtained. It is also difficult to guarantee a sufficientvolume fraction of retained austenite, and good ductility is notobtained. On the other hand, if C is excessively added to the steelbeyond 0.250%, hard martensite excessively increases in area ratio,which causes more microvoids at grain boundaries of martensite andfacilitates propagation of cracks during bend test and hole expansiontest, leading to a reduction in bendability and stretch flangeability.If excessive C is added to steel, hardening of welds and theheat-affected zone (HAZ) becomes significant and the mechanicalproperties of the welds deteriorate, leading to a reduction in spotweldability, arc weldability, and the like. From these perspectives, theC content is 0.030% or more and 0.250% or less. The C content ispreferably 0.080% or more. The C content is preferably 0.200% or less.

Si: 0.01% or More and 3.00% or Less

Si is an element that improves the strain hardenability of ferrite, andis thus a useful element for ensuring good ductility. If the Si contentis below 0.01%, the addition effect is limited. Thus the lower limit is0.01%. On the other hand, excessively adding Si beyond 3.00% not onlyembrittles the steel, but also causes red scales or the like todeteriorate surface characteristics. Therefore, the Si content is 0.01%or more and 3.00% or less. The Si content is preferably 0.20% or more.The Si content is preferably 2.00% or less.

Mn: 2.60% or More and 4.20% or Less

Mn is one of the very important elements for the disclosure. Mn is anelement that stabilizes retained austenite, and is thus a useful elementfor ensuring good ductility. Mn can also increase the TS of the steelthrough solid solution strengthening. These effects can be obtained whenthe Mn content in the steel is 2.60% or more. On the other hand,excessively adding Mn beyond 4.20% results in a rise in cost. From theseperspectives, the Mn content is 2.60% or more and 4.20% or less. The Mncontent is preferably 3.00% or more. The Mn content is preferably 4.20%or less.

P: 0.001% or More and 0.100% or Less

P is an element that has a solid solution strengthening effect and canbe added depending on the desired TS. P also facilitates ferritetransformation, and thus is also a useful element for forming amulti-phase structure in the steel sheet. To obtain this effect, the Pcontent in the steel sheet needs to be 0.001 or more. However, if the Pcontent exceeds 0.100%, weldability degrades and, when a galvanizedlayer is subjected to alloying treatment, the alloying rate decreases,impairing galvanizing quality. Therefore, the P content is 0.001% ormore and 0.100% or less. The P content is preferably 0.005% or more. TheP content is preferably 0.050% or less.

S: 0.0200% or Less

S segregates to grain boundaries, embrittles the steel during hotworking, and forms sulfides to reduce the local deformability of thesteel sheet. Therefore, the S content is 0.0200% or less, preferably0.0100% or less, and more preferably 0.0050% or less. Under productionconstraints, however, the S content is preferably 0.0001% or more.Therefore, the S content is preferably 0.0001% or more and 0.0200% orless. The S content is more preferably 0.0001% or more. The S content ismore preferably 0.0100% or less. The S content is further preferably0.0001% or more. The S content is further preferably 0.0050% or less.

N: 0.0100% or Less

N is an element that deteriorates the anti-aging property of the steel.The deterioration in anti-aging property becomes more pronounced,particularly when the N content exceeds 0.0100%. Accordingly, smaller Ncontents are more preferable. However, under production constraints, theN content is preferably 0.0005% or more. Therefore, the N content ispreferably 0.0005% or more and 0.0100% or less. The N content is morepreferably 0.0010% or more. The N content is more preferably 0.0070% orless.

Ti: 0.005% or More and 0.200% or Less

Ti is one of the very important elements for the disclosure. Ti isuseful for achieving strengthening by precipitation of the steel. Ti canalso ensure a desired area ratio of non-recrystallized ferrite, andcontributes to increasing the yield ratio of the steel sheet.Additionally, making use of relatively hard non-recrystallized ferrite,Ti can reduce the difference in hardness from a hard secondary phase(martensite or retained austenite), and also contributes to improvingstretch flangeability. These effects can be obtained when the Ti contentis 0.005% or more. On the other hand, if the Ti content in the steelexceeds 0.200%, hard martensite excessively increases in area ratio,which causes more microvoids at grain boundaries of martensite andfacilitates propagation of cracks during bend test and hole expansiontest, leading to a reduction in the bendability and stretchflangeability of the steel sheet. Therefore, the Ti content is 0.005% ormore and 0.200% or less. The Ti content is preferably 0.010% or more.The Ti content is preferably 0.100% or less.

The basic components according to this disclosure have been describedabove. The balance other than the components described above is Fe andinevitable impurities. Additionally, the following elements may b eoptionally contained as appropriate.

The chemical composition of the steel may further contain at least oneselected from the group consisting of Al: 0.01% or more and 2.00% orless, Nb: 0.005% or more and 0.200% or less, B: 0.0003% or more and0.0050% or less, Ni: 0.005% or more and 1.000% or less, Cr: 0.005% ormore and 1.000% or less, V: 0.005% or more and 0.500% or less, Mo:0.005% or more and 1.000% or less, Cu: 0.005% or more and 1.000% orless, Sn: 0.002% or more and 0.200% or less, Sb: 0.002% or more and0.200% or less, Ta: 0.001% or more and 0.010% or less, Ca: 0.0005% ormore and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, andREM: 0.0005% or more and 0.0050% or less.

Al is a useful element for increasing the area of a ferrite-austenitedual phase region and reducing annealing temperature dependency, i.e.,increasing the stability of the steel sheet as a material. In addition,Al acts as a deoxidizer, and is also a useful element for maintainingthe cleanliness of the steel. If the Al content is below 0.01%, however,the addition effect is limited. Thus the lower limit is 0.01%. On theother hand, excessively adding Al beyond 2.00% increases the risk ofcracking occurring in a semi-finished product during continuous casting,and inhibits manufacturability. From these perspectives, the Al contentis 0.01% or more and 2.00% or less. The Al content is preferably 0.20%or more. The Al content is preferably 1.20% or less.

Nb is useful for achieving strengthening by precipitation of the steel.The addition effect can be obtained when the content is 0.005% or more.Nb can also ensure a desired area ratio of non-recrystallized ferrite,as in the case of adding Ti, and contributes to increasing the yieldratio of the steel sheet. Additionally, making use of relatively hardnon-recrystallized ferrite, Nb can reduce the difference in hardnessfrom a hard secondary phase (martensite or retained austenite), and alsocontributes to improving stretch flangeability. On the other hand, ifthe Nb content in the steel exceeds 0.200%, hard martensite excessivelyincreases in area ratio, which causes more microvoids at grainboundaries of martensite and facilitates propagation of cracks duringbend test and hole expansion test. This leads to a reduction in thebendability and stretch flangeability of the steel sheet. This alsoincreases cost. Therefore, when added to steel, the Nb content is 0.005%or more and 0.200% or less. The Nb content is preferably 0.010% or more.The Nb content is preferably 0.100% or less.

B may be added as necessary, since it has the effect of suppressing thegeneration and growth of ferrite from austenite grain boundaries andenables microstructure control according to the circumstances. Theaddition effect can be obtained when the B content is 0.0003% or more.If the B content exceeds 0.0050%, however, the formability of the steelsheet degrades. Therefore, when added to steel, the B content is 0.0003%or more and 0.0050% or less. The B content is preferably 0.0005% ormore. The B content is preferably 0.0030% or less.

Ni is an element that stabilizes retained austenite, and is thus auseful element for ensuring good ductility, and that increases the TS ofthe steel through solid solution strengthening. The addition effect canbe obtained when the Ni content is 0.005% or more. On the other hand, ifthe Ni content in the steel exceeds 1.000%, hard martensite excessivelyincreases in area ratio, which causes more microvoids at grainboundaries of martensite and facilitates propagation of cracks duringbend test and hole expansion test. This leads to a reduction in thebendability and stretch flangeability of the steel sheet. This alsoincreases cost. Therefore, when added to steel, the Ni content is 0.005%or more and 1.000% or less.

Cr, V, and Mo are elements that may be added as necessary, since theyhave the effect of improving the balance between TS and ductility. Theaddition effect can be obtained when the Cr content is 0.005% or more,the V content is 0.005% or more, and/or the Mo content is 0.005% ormore. However, if the Cr content exceeds 1.000%, the V content exceeds0.500%, and/or the Mo content exceeds 1.000%, hard martensiteexcessively increases in area ratio, which causes more microvoids atgrain boundaries of martensite and facilitates propagation of cracksduring bend test and hole expansion test. This leads to a reduction inthe bendability and stretch flangeability of the steel sheet, and alsocauses a rise in cost. Therefore, when added to steel, the Cr content is0.005% or more and 1.000% or less, the V content is 0.005% or more and0.500% or less, and/or the Mo content is 0.005% or more and 1.000% orless.

Cu is a useful element for strengthening of steel and may be added forstrengthening of steel, as long as the content is within the rangedisclosed herein. The addition effect can be obtained when the Cucontent is 0.005% or more. On the other hand, if the Cu content in thesteel exceeds 1.000%, hard martensite excessively increases in arearatio, which causes more microvoids at grain boundaries of martensiteand facilitates propagation of cracks during bend test and holeexpansion test. This leads to a reduction in the bendability and stretchflangeability of the steel sheet. Therefore, when added to steel, the Cucontent is 0.005% or more and 1.000% or less.

Sn and Sb are elements that may be added as necessary from theperspective of suppressing decarbonization of a region extending fromthe surface layer of the steel sheet to a depth of about several tens ofmicrometers, which results from nitriding and/or oxidation of the steelsheet surface. Suppressing nitriding and/or oxidation in this way isuseful for preventing a reduction in the area ratio of martensite in thesteel sheet surface, and for ensuring the TS and stability of the steelsheet as a material. However, excessively adding Sn or Sb beyond 0.200%reduces toughness. Therefore, when Sn and/or Sb is added to steel, thecontent of each added element is 0.002% or more and 0.200% or less.

Ta forms alloy carbides or alloy carbonitrides, and contributes toincreasing the strength of the steel, as is the case with Ti and Nb. Itis also believed that Ta has the effect of effectively suppressingcoarsening of precipitates when partially dissolved in Nb carbides or Nbcarbonitrides to form complex precipitates, such as (Nb, Ta) (C, N), andproviding a stable contribution to increasing the strength of the steelsheet through strengthening by precipitation. Therefore, Ta ispreferably added to the steel according to the disclosure. The additioneffect of Ta can be obtained when the Ta content is 0.001% or more.Excessively adding Ta, however, fails to increase the addition effect,but instead results in a rise in alloying cost. Therefore, when added tosteel, the Ta content is 0.001% or more and 0.010% or less.

Ca, Mg, and REM are useful elements for causing spheroidization ofsulfides and mitigating the adverse effect of sulfides on hole expansionformability (stretch flangeability). To obtain this effect, it isnecessary to add any of these elements to steel in an amount of 0.0005%or more. However, if the content of each added element exceeds 0.0050%,more inclusions occur, for example, and some defects such as surfacedefects and internal defects are caused in the steel sheet. Therefore,when Ca, Mg, and/or REM is added to steel, the content of each addedelement is 0.0005% or more and 0.0050% or less.

The following provides a description of the microstructure. Sufficientductility of the steel sheet can be ensured by facilitating theformation of polygonal ferrite in the microstructure. This, however,causes decreases in tensile strength and yield strength. Besides, thersemechanical properties also vary depending on the area ratio ofmartensite, and the ductility is greatly affected by the amount ofretained austenite. Hence, the mechanical properties of thehigh-strength steel sheet can be effectively obtained by controlling theamounts (area ratio, volume fraction) of these phases (microstructures).As a result of conducting studies from this perspective, we newlydiscovered that the area ratios of polygonal ferrite andnon-recrystallized ferrite are controllable by the rolling reduction incold rolling. We also found out that the area ratio of martensite andthe volume fraction of retained austenite are mainly determined by theaddition amount of Mn. We further found out that, by omitting coldrolling or by limiting the rolling reduction in cold rolling to 30% orless, not only the area ratio of polygonal ferrite is reduced (i.e. canbe controlled to an appropriate range) (relative to the wholemicrostructure), but also the microstructure shape of the final productchanges greatly, yielding a steel sheet having crystal grains with ahigh aspect ratio. The value of hole expansion formability X is thusimproved. In detail, the microstructure of a steel sheet with highductility and favorable hole expansion formability is as follows.

Area Ratio of Polygonal Ferrite: 20% or More and 65% or Less

According to the disclosure, the area ratio of polygonal ferrite needsto be 20% or more to ensure sufficient ductility. On the other hand, toguarantee a TS of 590 MPa or more, the area ratio of soft polygonalferrite needs to be 65% or less. The area ratio of polygonal ferrite ispreferably 30% or more. The area ratio of polygonal ferrite ispreferably 55% or less. As used herein, “polygonal ferrite” refers toferrite that is relatively soft and that has high ductility.

Area Ratio of Non-Recrystallized Ferrite: 8% or More

In this disclosure, it is very important to set the area ratio ofnon-recrystallized ferrite to be 8% or more. In this regard,non-recrystallized ferrite is useful for increasing the strength of thesteel sheet. However, non-recrystallized ferrite may cause a significantdecrease in the ductility of the steel sheet, and thus is normallyreduced in a general process. In contrast, according to the presentdisclosure, by using polygonal ferrite and retained austenite to providegood ductility and intentionally utilizing relatively hardnon-recrystallized ferrite, it is possible to provide the steel sheetwith the intended TS, without having to form a large amount ofmartensite, such as exceeding 25% in area ratio.

Moreover, according to the present disclosure, interfaces betweendifferent phases, namely, between polygonal ferrite and martensite, arereduced, making it possible to increase the yield point (YP) and YR ofthe steel sheet.

To obtain these effects, the area ratio of non-recrystallized ferriteneeds to be 8% or more, preferably 10% or more. As used herein,“non-recrystallized ferrite” refers to ferrite that contains strain inthe grains with a crystal orientation difference of less than 15°, andthat is harder than the above-described polygonal ferrite with highductility.

In the disclosure, no upper limit is placed on the area ratio ofnon-recrystallized ferrite, yet a preferred upper limit is around 45%,considering the possibility of increased material anisotropy in thesteel sheet surface.

Area Ratio of Martensite: 5% or More and 25% or Less

To achieve TS of 590 MPa or more, the area ratio of martensite needs tobe 5% or more. On the other hand, to ensure good ductility, the arearatio of martensite needs to be limited to 25% or less.

According to the disclosure, the area ratios of ferrite (includingpolygonal ferrite and non-recrystallized ferrite) and martensite can bedetermined in the following way.

Specifically, a cross section of a steel sheet that is taken in thesheet thickness direction to be parallel to the rolling direction (whichis an L-cross section) is polished, then etched with 3 vol.% nital, andten locations are observed at 2000 times magnification under an SEM(scanning electron microscope), at a position of sheet thickness x 1/4(which is the position at a depth of one-fourth of the sheet thicknessfrom the steel sheet surface), to capture microstructure micrographs.The captured microstructure micrographs are used to calculate the arearatios of respective phases (ferrite and martensite) for the tenlocations using Image-Pro manufactured by Media Cybernetics, the resultsare averaged, and each average is used as the area ratio of thecorresponding phase. In the microstructure micrographs, polygonalferrite and non-recrystallized ferrite appear as a gray structure (basesteel structure), while martensite as a white structure.

According to the disclosure, the area ratios of polygonal ferrite andnon-recrystallized ferrite can be determined in the following way.Specifically, low-angle grain boundaries in which the crystalorientation difference is from 2° to less than 15° and large-angle grainboundaries in which the crystal orientation difference is 15° or moreare identified using EBSD (Electron Backscatter Diffraction). An IQ Mapis then created, considering ferrite that contains low-angle grainboundaries in the grains as non-recrystallized ferrite. Then, low-anglegrain boundaries and large-angle grain boundaries are extracted from thecreated IQ Map at ten locations, respectively, to determine the areas oflow-angle grain boundaries and large-angle grain boundaries at the tenlocations. Based on the results, the areas of polygonal ferrite andnon-recrystallized ferrite are calculated to determine the area ratiosof polygonal ferrite and non-recrystallized ferrite for the tenlocations. By averaging the results, the above-described area ratios ofpolygonal ferrite and non-recrystallized ferrite are determined.

Volume Fraction of Retained Austenite: 8% or More

According to the disclosure, the volume fraction of retained austeniteneeds to be 8% or more, and is preferably 10% or more, to ensuresufficient ductility. According to the disclosure, no upper limit isplaced on the area ratio of retained austenite, yet a preferred upperlimit is around 40%, considering the risk of formation of increasedamounts of unstable retained austenite resulting from insufficientconcentration of C, Mn, and the like, which is less effective inimproving ductility.

The volume fraction of retained austenite is calculated by determiningthe x-ray diffraction intensity of a plane of sheet thickness×¼ (whichis the plane at a depth of one-fourth of the sheet thickness from thesteel sheet surface), which is exposed by polishing the steel sheetsurface to a depth of one-fourth of the sheet thickness. Using anincident x-ray beam of MoKa, the intensity ratio of the peak integratedintensity of the {111}, {200}, {220}, and {311} planes of retainedaustenite to the peak integrated intensity of the {110}, {200}, and{211} planes of ferrite is calculated for all of the twelvecombinations, the results are averaged, and the average is used as thevolume fraction of retained austenite.

Average Grain Size of Polygonal Ferrite: 6 μm or Less

Refinement of polygonal ferrite grains contributes to improving YP andTS. Thus, to ensure a high YP and a high YR as well as a desired TS,polygonal ferrite needs to have an average grain size of 6 μm or less,and preferably 5 μm or less.

According to the disclosure, no lower limit is placed on the averagegrain size of polygonal ferrite, yet, from an industrial perspective, apreferred lower limit is around 0.3 μm.

Average Grain Size of Martensite: 3 μm or Less

Refinement of martensite grains contributes to improving bendability andstretch flangeability (hole expansion formability). Thus, to ensure highbendability and high stretch flangeability (high hole expansionformability), the average grain size of martensite needs to be limitedto 3 μm or less, and preferably to 2.5 μm or less.

According to the disclosure, no lower limit is placed on the averagegrain size of martensite, yet, from an industrial perspective, apreferred lower limit is around 0.1 μm.

Average Grain Size of Retained Austenite: 3 μm or Less

Refinement of retained austenite grains contributes to improvingductility, as well as bendability and stretch flangeability (holeexpansion formability). Accordingly, to ensure good ductility,bendability, and stretch flangeability (hole expansion formability) ofthe steel sheet, the average grain size of retained austenite needs tobe 3 μm or less, and preferably 2.5 μm or less. According to thedisclosure, no lower limit is placed on the average grain size ofretained austenite, yet, from an industrial perspective, a preferredlower limit is around 0.1 μm.

The average grain sizes of polygonal ferrite, martensite, and retainedaustenite are respectively determined by averaging the results fromcalculating equivalent circular diameters from the areas of polygonalferrite grains, martensite grains, and retained austenite grainsmeasured with Image-Pro as mentioned above. Polygonal ferrite,non-recrystallized ferrite, martensite, and retained austenite areseparated using EBSD, and martensite and retained austenite areidentified using an EBSD phase map. In this case, each of theabove-described average grain sizes is determined from the measurementsfor grains with a grain size of 0.01 μm or more. The reason is thatgrains with a grain size of less than 0.01 μm have no effect on thedisclosure.

Average Aspect Ratio of Crystal Grains of each of Polygonal ferrite,Martensite, and Retained Austenite: 2.0 or More and 15.0 or Less

In this disclosure, it is very important to set the average aspect ratioof crystal grains of each of polygonal ferrite, martensite, and retainedaustenite to 2.0 or more.

A lower aspect ratio of crystal grains indicates that, during retentionin heat treatment after cold rolling (cold-rolled sheet annealing),ferrite and austenite recover and recrystallize and then undergo graingrowth, resulting in the formation of crystal grains close to equiaxedgrains. The ferrite formed here is soft. In the case where cold rollingis omitted or the rolling reduction in cold rolling is less than 30%, onthe other hand, the amount of strain applied decreases, so that theformation of polygonal ferrite is suppressed and a microstructure mainlycomposed of crystal grains with a high aspect ratio results. Such amicrostructure composed of crystal grains with a high aspect ratio ishard because it contains a large amount of strain or has parts where thedistance between grain boundaries is short, as compared with theabove-mentioned microstructure. Therefore, not only the TS is improved,but also the difference in hardness from hard phases such as retainedaustenite and martensite decreases, and the hole expansion formabilityis improved without loss of ductility. If the aspect ratio is more than15.0, the TS increases extremely, and favorable ductility cannot beachieved.

Thus, the average aspect ratio of crystal grains of each of polygonalferrite, martensite, and retained austenite is limited to 2.0 or moreand 15.0 or less. In terms of ductility, the average aspect ratio ismore preferably 2.2 or more, and more preferably 2.4 or more.

The aspect ratio of a crystal grain mentioned here is a value obtainedby dividing the major axis length of the crystal grain by the minor axislength of the crystal grain. The average aspect ratio of each type ofcrystal grains can be calculated as follows.

For each of polygonal ferrite grains, martensite grains, and retainedaustenite grains, the major axis length and minor axis length of each of30 crystal grains are calculated using the above-mentioned Image-Pro,the major axis length is divided by the minor axis length, and thedivision results are averaged.

A Value Obtained by Dividing the Mn Content in the Retained Austenite(in mass %) by the Mn Content in the Polygonal Ferrite (in mass %): 2.0or More

In this disclosure, it is very important that the value obtained bydividing the Mn content in the retained austenite (in mass %) by the Mncontent in the polygonal ferrite (in mass %) equals 2.0 or more. Thereason is that better ductility requires a larger amount of stableretained austenite with concentrated Mn.

According to the disclosure, no upper limit is placed on the valueobtained by dividing the Mn content in the retained austenite (in mass%) by the Mn content in the polygonal ferrite (in mass %), yet apreferred upper limit is around 16.0 from the perspective of ensuringstretch flangeability.

The Mn content in the retained austenite (in mass %) and the Mn contentin the polygonal ferrite (in mass %) can be determined in the followingway.

Specifically, an EPMA (Electron Probe Micro Analyzer) is used toquantify the distribution of Mn in each phase in a cross section alongthe rolling direction at a position of sheet thickness×¼. Then, 30retained austenite grains and 30 ferrite grains are analyzed todetermine Mn contents, the results are averaged, and each average isused as the Mn content in the corresponding phase.

In addition to the above-described polygonal ferrite, martensite, and soon, the microstructure according to the disclosure may further includecarbides ordinarily found in steel sheets, such as granular ferrite,acicular ferrite, bainitic ferrite, tempered martensite, pearlite, andcementite (excluding cementite in pearlite). Any of these structures maybe included as long as the area ratio is 10% or less, without impairingthe effect of the disclosure.

We made further investigations on the microstructures of steel sheetsupon performing press forming and working.

As a result, it was discovered that there are two types of retainedaustenite: one transforms to martensite immediately upon the subjectionof the steel sheet to press forming or working, while the other persistsuntil the working ratio becomes high enough to cause the retainedaustenite to eventually transform to martensite, bringing about a TRIPphenomenon (transformation induced plasticity phenomenon). It was alsorevealed that good elongation can be obtained in a particularlyeffective way when a large amount of retained austenite transforms tomartensite after the working ratio becomes high enough.

Specifically, as a result of collecting samples with good and poorelongation and measuring the quantity of retained austenite by varyingthe degree of tensile working from 0% to 20%, the working ratio and thequantity of retained austenite showed a tendency as illustrated inFIG. 1. As used herein, “the working ratio” refers to the elongationratio that is determined from a tensile test performed on a JIS No. 5test piece sampled from a steel sheet with the tensile direction beingperpendicular to the rolling direction of the steel sheet.

It can be seen from FIG. 1 that the samples with good elongation eachshowed a gentle decrease in the quantity of retained austenite as theworking ratio increased.

Accordingly, we further measured the quantity of retained austenite ineach sample with TS of 780 MPa after subjection to tensile working withan elongation value of 10%, and examined the effect of the ratio of thequantity of retained austenite after the tensile working to the quantitybefore the tensile working on the total elongation of the steel sheet.The results are shown in FIG. 2.

It can be seen from FIG. 2 that elongation is good if the value obtainedby dividing the volume fraction of retained austenite remaining in asteel after subjection to tensile working with an elongation value of10% by the volume fraction of retained austenite before the tensileworking equals 0.3 or more, but otherwise elongation is poor.

Therefore, it is preferable in the disclosure that the value obtained bydividing the volume fraction of retained austenite remaining in a steelafter subjection to tensile working with an elongation value of 10% bythe volume fraction of retained austenite before the tensile workingequals 0.3 or more. The reason is that this set up may ensure thetransformation of sufficient retained austenite to martensite after theworking ratio becomes high enough.

The above-described TRIP phenomenon requires retained austenite to bepresent before performing press forming or working. To cause retainedaustenite to be present before performing press forming or working, theMs point (martensite transformation start temperature) which depends onthe elements contained in the steel microstructure needs to be as low asapproximately 15° C. or lower.

Specifically, in the tensile working with an elongation value of 10%according to the disclosure, a tensile test is performed on a JIS No. 5test piece sampled from a steel sheet with the tensile direction beingperpendicular to the rolling direction of the steel sheet, and the testis interrupted when the elongation ratio reaches 10%, thus applyingtensile working with an elongation value of 10% to the test piece.

The volume fraction of retained austenite can be determined in theabove-described way.

Upon a detailed study of samples satisfying the above conditions, wediscovered that a TRIP phenomenon providing high strain hardenabilityoccurs upon working and even better elongation can be achieved if the Ccontent and the Mn content in the retained austenite satisfy thefollowing relation:

0.09*[Mn]−0.026−0.150≤[C]≤0.09*[Mn]−0.026+0.150

where

[C content] is the C content in the retained austenite in mass %, and

[Mn content] is the Mn content in the retained austenite in mass %.

When the above requirements are met, it is possible to cause a TRIPphenomenon, which is a key factor of improving ductility, to occurintermittently up until the final stage of working performed on thesteel sheet, guaranteeing the generation of so-called stable retainedaustenite.

The C content in the retained austenite (in mass %) can be determined inthe following way.

Specifically, an EPMA is used to quantify the distribution of C in eachphase in a cross section along the rolling direction at a position ofsheet thickness×¼. Then, 30 retained austenite grains are analyzed todetermine C contents, the results are averaged, and the average is usedas the C content.

Note that the Mn content in the retained austenite (in mass %) can bedetermined in the same way as the C content in the retained austenite.

The following describes the production conditions.

Steel Slab Heating Temperature: 1100° C. or Higher and 1300° C. or Lower

Precipitates that are present at the time of heating of a steel slab(hereinafter, also referred to simply as a “slab”) will remain as coarseprecipitates in the resulting steel sheet, making no contribution tostrength. Thus, remelting of any Ti- and Nb-based precipitates formedduring casting is required.

In this respect, if a steel slab is heated at a temperature below 1100°C., it is difficult to cause sufficient dissolution of carbides, leadingto problems such as an increased risk of trouble during the hot rollingresulting from increased rolling load. Therefore, the steel slab heatingtemperature is preferably 1100° C. or higher.

In addition, from the perspective of obtaining a smooth steel sheetsurface by scaling-off defects in the surface layer of the slab, such asblow hole generation, segregation, and the like, and reducing cracks andirregularities over the steel sheet surface, the steel slab heatingtemperature is preferably 1100° C. or higher.

If the steel slab heating temperature exceeds 1300° C., however, scaleloss increases as oxidation progresses. Therefore, the steel slabheating temperature is preferably 1300° C. or lower. For this reason,the steel slab heating temperature is preferably 1100° C. or higher and1300° C. or lower. The steel slab heating temperature is furtherpreferably 1150° C. or higher. The steel slab heating temperature isfurther preferably 1250° C. or lower.

A steel slab is preferably made with continuous casting to prevent macrosegregation, yet may be produced with other methods such as ingotcasting or thin slab casting. The steel slab thus produced may be cooledto room temperature and then heated again according to a conventionalprocess. Moreover, energy-saving processes are applicable without anyproblem, such as hot direct rolling or direct rolling in which either awarm steel slab without being fully cooled to room temperature ischarged into a heating furnace, or a steel slab is hot rolledimmediately after being subjected to heat retaining for a short period.A steel slab is subjected to rough rolling under normal conditions andformed into a sheet bar. When the heating temperature is low, it ispreferable to additionally heat the sheet bar using a bar heater or thelike prior to finish rolling, from the viewpoint of preventing troublesduring the hot rolling.

Finisher delivery temperature in hot rolling: 750° C. or higher and1000° C. or lower

The heated steel slab is hot rolled through rough rolling and finishrolling to form a hot-rolled sheet. At this point, when the finisherdelivery temperature exceeds 1000° C., the amount of oxides (scales)generated suddenly increases and the interface between the steelsubstrate and oxides becomes rough, which tends to lower the surfacequality of the steel sheet after subjection to pickling and coldrolling. In addition, any hot rolling scales persisting after picklingadversely affect the ductility and stretch flangeability of the steelsheet. Moreover, grain size is excessively coarsened, causing surfacedeterioration in a pressed part during working. On the other hand, ifthe finisher delivery temperature is below 750° C., rolling loadincreases and rolling is performed more often with austenite being in anon-recrystallized state. As a result, an abnormal texture develops inthe steel sheet, and the final product has a significant planaranisotropy such that the material properties not only become lessuniform (the stability as a material decreases), but the ductilityitself also deteriorates. Besides, if the finisher delivery temperaturein the hot rolling is lower than 750° C. or higher than 1000° C., amicrostructure having 8% or more of retained austenite in volumefraction cannot be obtained.

Therefore, the finisher delivery temperature in the hot rolling needs tobe 750° C. or higher and 1000° C. or lower. The finisher deliverytemperature is preferably 800° C. or higher. The finisher deliverytemperature is preferably 950° C. or lower.

Average Coiling Temperature after Hot Rolling: 300° C. or Higher and750° C. or Lower

When the average coiling temperature after the hot rolling is above 750°C., the grain size of ferrite in the microstructure of the hot-rolledsheet increases, making it difficult to ensure a desired strength of thefinal-annealed sheet. Besides, when the average coiling temperatureafter the hot rolling is above 750° C., a microstructure with an averagegrain size of polygonal ferrite of 6 μm or less, an average grain sizeof martensite of 3 μm or less, and an average grain size of retainedaustenite of 3 μm or less cannot be obtained. On the other hand, whenthe average coiling temperature after the hot rolling is below 300° C.,there is an increase in the strength of the hot-rolled sheet and in therolling load for cold rolling, and the steel sheet suffers malformation.As a result, productivity decreases. Therefore, the average coilingtemperature after the hot rolling needs to be 300° C. or higher and 750°C. or lower. The average coiling temperature is preferably 400° C. orhigher. The average coiling temperature is preferably 650° C. or lower.

According to the disclosure, finish rolling may be performedcontinuously by joining rough-rolled sheets during the hot rolling.Rough-rolled sheets may be coiled on a temporary basis. At least part offinish rolling may be conducted as lubrication rolling to reduce therolling load during the hot rolling. Conducting lubrication rolling insuch a manner is effective from the perspective of making the shape andmaterial properties of the steel sheet uniform. In lubrication rolling,the coefficient of friction is preferably 0.10 or more. The coefficientof friction is preferably 0.25 or less.

The hot-rolled sheet thus produced is subjected to pickling. Picklingenables removal of oxides from the steel sheet surface, and is thusimportant to ensure that the high-strength steel sheet as the finalproduct has good chemical convertibility and sufficient coating quality.The pickling may be performed in one or more batches.

Hot Band Annealing (First Heat Treatment): to Retain in a TemperatureRange of [Ac₁ Transformation Temperature+20° C.] to [Ac₁ TransformationTemperature+120° C.] for 600 s to 21,600 s

In this disclosure, it is very important to retain the steel sheet in atemperature range of [Ac₁ transformation temperature+20° C.] to [Ac₁transformation temperature+120° C.] for 600 s to 21,600 s.

If the hot band annealing is performed at an annealing temperature below[Ac₁ transformation temperature+20° C.] or above [Ac₁ transformationtemperature+120° C], or if the holding time is shorter than 600 s,concentration of Mn in austenite does not proceed in either case, makingit difficult to ensure a sufficient volume fraction of retainedaustenite after the final annealing. As a result, ductility decreases.Besides, a microstructure in which the value obtained by dividing the Mncontent in retained austenite (in mass %) by the Mn content in polygonalferrite (in mass %) equals 2.0 or more cannot be obtained. On the otherhand, if the steel sheet is retained for more than 21,600 s,concentration of Mn in austenite reaches a plateau, and becomes lesseffective in improving ductility after the final annealing, resulting ina rise in costs.

Therefore, in the hot band annealing (first heat treatment) according tothe disclosure, the steel sheet is retained in a temperature range of[Ac₁ transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 s.

The above-described heat treatment process may be continuous annealingor batch annealing. After the above-described heat treatment, the steelsheet is cooled to room temperature. The cooling process and coolingrate are not particularly limited, however, and any type of cooling maybe performed, including furnace cooling and air cooling in batchannealing and gas jet cooling, mist cooling, and water cooling incontinuous annealing. The pickling may be performed according to aconventional process.

Annealing (Second Heat Treatment): to Retain in a Temperature Range of[Ac₁ Transformation Temperature+10° C.] to [Ac₁ TransformationTemperature+100° C.] for 20 s to 900 s

In this disclosure, it is very important to retain the steel sheet in atemperature range of [Ac₁ transformation temperature+10° C.] to [Ac₁transformation temperature+100° C.] for 20 s to 900 s. When theannealing temperature is below [Ac₁ transformation temperature+10° C.]or above [Ac₁ transformation temperature+100° C], or if the holding timeis shorter than 20 s, concentration of Mn in austenite does not proceedin either case, making it difficult to ensure a sufficient volumefraction of retained austenite. As a result, ductility decreases.Besides, a microstructure in which the value obtained by dividing the Mncontent in retained austenite (in mass %) by the Mn content in polygonalferrite (in mass %) equals 2.0 or more cannot be obtained. On the otherhand, if the steel sheet is retained for more than 900 s, the area ratioof non-crystallized ferrite decreases and the interfaces betweendifferent phases, namely, between ferrite and hard secondary phases(martensite and retained austenite), are reduced, leading to a reductionin both YP and YR. Besides, a microstructure with an average grain sizeof martensite of 3μm or less and an average grain size of retainedaustenite of 3 μm or less cannot be obtained.

Rolling Reduction in Cold Rolling: Less than 30%

Cold rolling may be performed after the hot band annealing and beforethe annealing (second heat treatment). In this case, the rollingreduction needs to be less than 30%. By omitting the cold rolling orperforming the cold rolling with a rolling reduction of less than 30%,polygonal ferrite which forms by recrystallization after the heattreatment does not form and a microstructure elongated in the rollingdirection remains, and eventually polygonal ferrite, retained austenite,and martensite with a high aspect ratio are obtained. Thus, not only thestrength-ductility balance is improved, but also the stretchflangeability (hole expansion formability) is improved. If the rollingreduction is 30% or more, a microstructure having 20% or more and 65% orless of polygonal ferrite in area ratio and a microstructure having anaverage aspect ratio of crystal grains of each of polygonal ferrite,martensite, and retained austenite of 2.0 or more and 15.0 or lesscannot be obtained.

Hot-Dip Galvanizing Treatment

In hot-dip galvanizing treatment according to the disclosure, the steelsheet subjected to the above-described annealing (second heat treatment)is dipped in a galvanizing bath at 440° C. or higher and 500° C. orlower for hot-dip galvanizing. Subsequently, the coating weight on thesteel sheet surface is adjusted using gas wiping or the like.Preferably, the hot-dip galvanizing is performed using a galvanizingbath containing 0.10 mass % or more and 0.22 mass % or less of Al.

Moreover, when a hot-dip galvanized layer is subjected to alloyingtreatment, the alloying treatment may be performed in a temperaturerange of 450° C. to 600° C. after the above-described hot-dipgalvanizing treatment. If the alloying treatment is performed at atemperature above 600° C., untransformed austenite transforms topearlite, where a desired volume fraction of retained austenite cannotbe ensured and ductility degrades. On the other hand, if the alloyingtreatment is performed at a temperature below 450° C., the alloyingprocess does not proceed, making it difficult to form an alloy layer.

Therefore, when the galvanized layer is subjected to alloying treatment,the alloying treatment is performed in a temperature range of 450° C. to600° C.

Although other manufacturing conditions are not particularly limited,the series of processes including the annealing, hot-dip galvanizing,and alloying treatment described above may preferably be performed in acontinuous galvanizing line (CGL), which is a hot-dip galvanizing line,from the perspective of productivity.

When hot-dip aluminum coating treatment is performed, the steel sheetsubjected to the above-described annealing treatment is dipped in analuminum molten bath at 660° C. to 730° C. for hot-dip aluminum coatingtreatment. Subsequently, the coating weight is adjusted using gas wipingor the like. If the steel sheet has a composition such that thetemperature of the aluminum molten bath falls within the temperaturerange of [Ac₁ transformation temperature+10° C.] to [Ac₁ transformationtemperature+100° C], the steel sheet is preferably subjected to hot-dipaluminum coating treatment because finer and more stable retainedaustenite can be formed, and therefore further improvement in ductilitycan be achieved.

Electrogalvanizing treatment

According to the disclosure, electrogalvanizing treatment may also beperformed on the steel sheet after the heat treatment. No particularlimitations are placed on the electrogalvanizing treatment conditions,yet the electrogalvanizing treatment conditions are preferably set sothat the plated layer has a thickness of 5 μm to 15 μm.

According to the disclosure, the above-described steel sheet, hot-dipgalvanized steel sheet, hot-dip aluminum-coated steel sheet, andelectrogalvanized steel sheet may be subjected to skin pass rolling forthe purposes of straightening, adjustment of roughness on the sheetsurface, and the like. The skin pass rolling is preferably performed ata rolling reduction of 0.1% or more. The skin pass rolling is preferablyperformed at a rolling reduction of 2.0% or less.

When the rolling reduction is less than 0.1%, the skin pass rollingbecomes less effective and more difficult to control. Thus, a preferablerange for the rolling reduction has a lower limit of 0.1%. On the otherhand, when the skin pass rolling is performed at a rolling reductionabove 2.0%, the productivity of the steel sheet decreases significantly.Thus, the preferable range for the rolling reduction has an upper limitof 2.0%.

The skin pass rolling may be performed on-line or off-line. Skin passmay be performed in one or more batches to achieve a target rollingreduction.

Moreover, the steel sheet, the hot-dip galvanized steel sheet, thehot-dip aluminum-coated steel sheet, and the electrogalvanized steelsheet according to the disclosure may be subjected to a variety ofcoating treatment options, such as those using coating of resin, fatsand oils, and the like.

EXAMPLES

Steels having the chemical compositions as presented in Table 1, withthe balance consisting of Fe and inevitable impurities, were prepared bysteelmaking in a converter, and formed into slabs through continuouscasting. The slabs thus obtained were formed into a variety of steelsheets, as described below, by varying the conditions as listed in Table2.

After being hot rolled, each steel sheet was annealed in a temperaturerange of [Ac₁ transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C]. After being cold rolled (or without cold rolling),each steel sheet was annealed in a temperature range of [Ac₁transformation temperature+10° C.] to [Ac₁ transformationtemperature+100° C]. Consequently, a cold-rolled steel sheet (CR) wasobtained, and subjected to coating treatment to form a hot-dipgalvanized steel sheet (GI), a galvannealed steel sheet (GA), a hot-dipaluminum-coated steel sheet (Al), an electrogalvanized steel sheet (EG),or the like.

Used as hot-dip galvanizing baths were a zinc bath containing 0.19 mass% of Al for hot-dip galvanized steel sheets (GI) and a zinc bathcontaining 0.14 mass % of Al for galvannealed steel sheets (GA). Ineither case, the bath temperature was 465° C. and the coating weight perside was 45 g/m² (in the case of both-sided coating). For GA, the Feconcentration in the coating layer was adjusted to be 9 mass % or moreand 12 mass % or less. The bath temperature of the hot-dip aluminummolten bath for hot-dip aluminum-coated steel sheets was set at 700° C.

For each of the steel sheets thus obtained, the cross-sectionalmicrostructure, tensile property, hole expansion formability,bendability, and the like were investigated. The results are listed inTables 3 to 5.

The Ac₁ transformation temperature was calculated by:

[Ac₁ transformation temperature (° C.)]=751−16*(% C)+11*(% Si)−28*(%Mn)−5.5*(% Cu)−16*(% Ni)+13*(% Cr)+3.4*(% Mo)

where (% C), (% Si), (% Mn), (% Ni), (% Cu), (% Cr), and (% Mo) eachrepresent the content in steel (in mass %) of the element in theparentheses.

Tensile test was performed in accordance with JIS Z 2241 (2011) tomeasure YP, YR, TS, and EL using JIS No. 5 test pieces, each of whichwas sampled in a manner that the tensile direction was perpendicular tothe rolling direction of the steel sheet. Note that YR is YP divided byTS, expressed as a percentage. In this case, the results were determinedto be good when YR 68% and when TS * EL≥24,000 MPa·%. Also, EL wasdetermined to be good when EL≥34% for TS 590 MPa grade, EL≥30% for TS780 MPa grade, and EL≥24% for TS 980 MPa grade. In this case, a steelsheet of TS 590 MPa grade refers to a steel sheet with TS of 590 MPa ormore and less than 780 MPa, a steel sheet of TS 780 MPa grade refers toa steel sheet with TS of 780 MPa or more and less than 980 MPa, and asteel sheet of TS 980 MPa grade refers to a steel sheet with TS of 980MPa or more and less than 1180 MPa.

Bend test was performed according to the V-block method specified in JISZ 2248 (1996). Each steel sheet was visually observed under astereoscopic microscope for cracks on the outside of the bent portion,and the minimum bending radius without cracks was used as the limitbending radius R. In this case, the bendability of the steel sheet wasdetermined to be good if the following condition was satisfied: limitbending radius R at 90° V-bending/t≤1.5 (where t is the thickness of thesteel sheet).

Hole expansion test was performed in accordance with JIS Z 2256 (2010).Each of the steel sheets obtained was cut to a size of 100 mm * 100 mm,and a hole of 10 mm in diameter was drilled through each sample withclearance 12%±1%. Then, each steel sheet was clamped into a die havingan inner diameter of 75 mm with a blank holding force of 9 tons (88.26kN). In this state, a conical punch of 60° was pushed into the hole, andthe hole diameter at the crack initiation limit was measured. Then, toevaluate hole expansion formability, the maximum hole expansion ratio(%) was calculated by:

Maximum hole expansion ratio λ(%)={(D_(f)-D₀)/D₀}*100

where D_(f) is a hole diameter at the time of occurrence of cracking(mm) and D₀ is an initial hole diameter (mm).

In this case, the maximum hole expansion ratio was determined to be goodwhen λ≥34% for TS 590 MPa grade, λ≥30% for TS 780 MPa grade, and λ≥25%for TS 980 MPa grade.

The sheet passage ability during hot rolling was determined to be lowwhen it was considered that the risk of troubles, such as malformationduring hot rolling due to increased rolling load, would increasebecause, for example, the hot-rolling finisher delivery temperature waslow and rolling would be performed more often with austenite being in anon-crystallized state, or rolling would be performed in anaustenite-ferrite dual phase region. The sheet passage ability duringcold rolling was determined to be low when it was considered that therisk of troubles, such as malformation during cold rolling due toincreased rolling load, would increase because, for example, the coilingtemperature during hot rolling was low and the hot-rolled sheet had asteel microstructure in which low-temperature transformation phases,such as bainite and martensite, were dominantly present.

The surface characteristics of each final-annealed sheet were determinedto be poor when defects such as blow hole generation and segregation onthe surface layer of the slab could not be scaled-off, cracks andirregularities on the steel sheet surface increased, and a smooth steelsheet surface could not be obtained. The surface characteristics of eachfinal-annealed sheet were also determined to be poor when the amount ofoxides (scales) generated suddenly increased, interfaces between thesteel substrate and oxides were roughened, and the surface quality afterpickling and cold rolling degraded, or when hot-rolling scales persistedat least in part after pickling.

In this case, productivity was evaluated according to the lead timecosts, including: (1) malformation of a hot-rolled sheet occurred; (2) ahot-rolled sheet requires straightening before proceeding to thesubsequent steps; and (3) a prolonged holding time during the annealingtreatment. The productivity was determined to be “high” when none of (1)to (3) applied and “low” when any of (1) to (3) applied.

Tensile working was performed in accordance with JIS Z 2241 (2011) usingJIS No. 5 test pieces, each of which was sampled in a manner that thetensile direction was perpendicular to the rolling direction of thesteel sheet. A value was obtained by dividing the volume fraction ofretained austenite remaining in each steel sheet after subjection totensile working with an elongation value of 10% by the volume fractionof retained austenite before the working (10% application). The volumefraction of retained austenite was measured in accordance with the aboveprocedure.

The measurement results are also listed in Table 4.

The C content in the retained austenite (in mass %) and the Mn contentin the retained austenite (in mass %) were measured in accordance withthe above procedure.

The measurement results are also listed in Table 4.

TABLE 1 Chemical composition (mass %) Steel sample ID C Si Mn P S N TiAl Nb B Ni Cr V Mo A 0.111 0.34 3.55 0.023 0.0022 0.0037 0.035 — — — — —— — B 0.156 0.65 3.91 0.028 0.0019 0.0035 0.038 — — — — — — — C 0.1701.24 4.11 0.024 0.0023 0.0033 0.035 — — — — — — — D 0.070 1.25 3.510.027 0.0020 0.0031 0.033 — — — — — — — E 0.155 0.80 3.78 0.031 0.00190.0035 0.033 — — — — — — — F 0.141 0.05 3.80 0.021 0.0025 0.0032 0.015 —— — — — — — G 0.198 0.87 3.78 0.025 0.0020 0.0041 0.022 — — — — — — — H0.166 0.74 2.87 0.025 0.0020 0.0034 0.035 — — — — — — — I 0.160 0.523.91 0.028 0.0025 0.0031 0.041 — — — — — — — IA 0.030 0.42 3.55 0.0270.0021 0.0031 0.041 — — — — — — — IB 0.152 3.00 3.87 0.028 0.0020 0.00320.042 — — — — — — — IC 0.115 0.52 2.60 0.021 0.0021 0.0035 0.035 — — — —— — — ID 0.156 0.51 2.99 0.001 0.0019 0.0034 0.035 — — — — — — — IE0.151 0.34 3.01 0.100 0.0021 0.0029 0.036 — — — — — — — IF 0.152 0.363.15 0.031 0.0001 0.0035 0.038 — — — — — — — IG 0.118 0.51 3.00 0.0340.0200 0.0031 0.039 — — — — — — — IH 0.124 0.45 3.25 0.025 0.0022 0.00050.041 — — — — — — — II 0.147 0.48 3.45 0.031 0.0023 0.0100 0.041 — — — —— — — IJ 0.142 0.35 3.69 0.028 0.0018 0.0031 0.005 — — — — — — — IK0.145 0.41 3.57 0.029 0.0025 0.0032 0.200 — — — — — — — IL 0.148 4.223.87 0.031 0.0025 0.0035 0.042 — — — — — — — IM 0.115 0.55 3.45 0.0310.0019 0.0034 0.210 — — — — — — — J 0.010 0.52 3.21 0.031 0.0021 0.00310.039 — — — — — — — K 0.199 4.51 2.51 0.028 0.0025 0.0038 0.024 — — — —— — — L 0.186 1.01 2.25 0.025 0.0025 0.0034 0.025 — — — — — — — M 0.1810.78 3.80 0.022 0.0021 0.0038 0.001 — — — — — — — N 0.205 0.89 3.510.024 0.0029 0.0038 0.033 0.45 — — — — — — O 0.200 1.02 3.78 0.0280.0025 0.0034 0.033 — 0.042 — — — — — P 0.191 0.83 3.54 0.027 0.00240.0041 0.035 — — 0.0015 — — — — Q 0.210 1.01 3.57 0.029 0.0023 0.00320.021 — — — 0.312 — — — R 0.212 0.55 4.11 0.029 0.0022 0.0031 0.025 — —— — 0.355 — — S 0.225 0.78 3.76 0.031 0.0023 0.0032 0.023 — — — — —0.035 — T 0.201 0.98 3.29 0.031 0.0022 0.0041 0.035 — — — — — — 0.329 U0.202 1.32 3.12 0.025 0.0025 0.0033 0.041 — — — — — — — V 0.189 1.023.55 0.026 0.0029 0.0034 0.045 — — — — — — — W 0.190 0.88 3.45 0.0250.0025 0.0035 0.033 — — — — — — — X 0.199 0.78 3.52 0.026 0.0025 0.00330.035 — 0.047 — — — — — Y 0.208 0.51 3.23 0.029 0.0027 0.0041 0.036 —0.032 — — — — — Z 0.204 0.29 3.65 0.027 0.0019 0.0031 0.034 — 0.038 — —— — — AA 0.199 1.01 3.87 0.025 0.0025 0.0037 0.037 — — — — — — — AB0.210 1.28 4.02 0.029 0.0029 0.0041 0.033 — — — — — — — AC 0.199 1.114.09 0.031 0.0021 0.0035 0.033 — — — — — — — AD 0.188 0.81 3.54 0.0280.0020 0.0039 0.026 — — — — — — — Ac₁ transformation Chemicalcomposition (mass %) temperature Steel sample ID Cu Sn Sb Ta Ca Mg REM(° C.) Remarks A — — — — — — — 654 Conforming steel B — — — — — — — 646Conforming steel C — — — — — — — 647 Conforming steel D — — — — — — —665 Conforming steel E — — — — — — — 651 Conforming steel F — — — — — —— 643 Conforming steel G — — — — — — — 652 Conforming steel H — — — — —— — 676 Conforming steel I — — — — — — — 645 Conforming steel IA — — — —— — — 656 Conforming steel IB — — — — — — — 673 Conforming steel IC — —— — — — — 670 Conforming steel ID — — — — — — — 670 Conforming steel IE— — — — — — — 668 Conforming steel IF — — — — — — — 664 Conforming steelIG — — — — — — — 671 Conforming steel IH — — — — — — — 663 Conformingsteel II — — — — — — — 657 Conforming steel IJ — — — — — — — 649Conforming steel IK — — — — — — — 653 Conforming steel IL — — — — — — —687 Comparative steel IM — — — — — — — 659 Comparative steel J — — — — —— — 667 Comparative steel K — — — — — — — 727 Comparative steel L — — —— — — — 696 Comparative steel M — — — — — — — 650 Comparative steel N —— — — — — — 659 Conforming steel O — — — — — — — 653 Conforming steel P— — — — — — — 658 Conforming steel Q — — — — — — — 654 Conforming steelR — — — — — — — 643 Conforming steel S — — — — — — — 651 Conformingsteel T — — — — — — — 668 Conforming steel U 0.259 — — — — — — 674Conforming steel V — 0.006 — — — — — 660 Conforming steel W — — — 0.005— — — 661 Conforming steel X — — — — — — — 658 Conforming steel Y — —0.007 — — — — 663 Conforming steel Z — — — 0.008 — — — 649 Conformingsteel AA — — — — 0.0021 — — 651 Conforming steel AB — — — — — 0.0021 —649 Conforming steel AC — — — — — — 0.0028 646 Conforming steel AD — — —— — — — 658 Conforming steel Underline: outside range according topresent disclosure

TABLE 2 Finisher Average hot-rolled sheet heat treatment Steel Slabheating delivery coiling Heat treatment Heat treatment sampletemperature temperature temperature temperature time No. ID (° C.) (°C.) (° C.) (° C.) (s) 1 A 1230 900 540 703 18000 2 B 1240 890 500 68820000 3 C 1200 890 600 690 15000 4 C 1100 890 600 690 20000 5 C 1300 880580 700 21000 6 C 1200 750 600 690 20000 7 C 1200 1000  680 710 18000 8C 1150 880 300 690 19000 9 C 1180 870 750 690 20000 10 C 1180 870 600670 19000 11 C 1190 860 550 760 18000 12 C 1200 850 600 690  600 13 C1210 850 540 680 17000 14 C 1200 890 540 680 17000 15 C 1250 870 660 69019000 16 C 1220 700 540 690 20000 17 C 1230 1100  490 690 10000 18 C1240 860 850 690  7000 19 C 1250 870 510 500 16000 20 C 1240 880 490 85017000 21 C 1250 890 570 690  300 22 C 1250 890 600 690 19000 23 C 1240890 600 690 20000 24 C 1230 860 610 690 19000 25 C 1230 860 610 690 6000 26 C 1240 880 590 690  6000 27 C 1210 880 560 690 10000 28 C 1230860 560 690 16000 29 C 1200 890 570 690 20000 30 D 1240 860 530 69618000 31 E 1240 890 520 689  6000 32 F 1250 900 550 690 18000 33 G 1240890 590 687  7000 34 H 1260 860 560 715  9000 35 I 1240 920 600 69115000 36 IA 1250 890 600 700 15000 37 IB 1250 890 600 710 18000 38 IC1240 890 560 750 18000 39 ID 1230 880 570 700 18000 40 IE 1250 890 580710 19000 41 IF 1240 860 560 710 18000 42 IG 1230 860 560 710 18000 43IH 1250 860 570 715 18000 44 II 1250 870 600 712 18000 45 IJ 1250 890580 720 18000 46 IK 1230 880 580 690 18000 47 IL 1240 870 590 710 1800048 IM 1250 870 570 710 18000 49 J 1220 860 630 717 17000 50 K 1210 870620 764 20000 51 L 1240 840 570 729 19000 52 M 1250 830 540 688 5000 53N 1260 850 580 684 6000 54 O 1270 870 540 695 15000 55 P 1220 900 520695 18000 56 Q 1260 840 600 696 19000 57 R 1270 830 560 683 14000 58 S1240 880 620 690 8000 59 T 1250 820 600 700 8000 60 U 1250 850 530 71515000 61 V 1240 920 570 692 13000 62 W 1230 910 500 705 10000 63 X 1250890 590 695 16000 64 Y 1260 900 520 702 9000 65 Z 1250 880 540 686 1800066 AA 1260 900 520 686 9000 67 AB 1250 880 540 694 15000 68 AC 1260 860530 686 6000 69 AD 1250 870 540 696 8000 70 B 1230 890 550 695 10000 71B 1250 870 530 698 8000 Annealing treatment Rolling reduction Heattreatment Heat treatment in cold rolling temperature time No. (%) (° C.)(s) Type* Remarks 1 27.6 688 500 CR Example 2 27.6 673 300 CR Example 328.6 675 350 GA Example 4 28.6 680 500 CR Example 5 28.6 680 350 CRExample 6 22.2 675 350 CR Example 7 29.6 690 400 GA Example 8 28.6 675400 GA Example 9 25.9 690 400 CR Example 10 25.9 700 350 CR Example 1128.6 690 500 CR Example 12 25.0 680 500 GA Example 13 25.0 740 500 GAExample 14 22.2 680  20 GA Example 15 28.6 670 900 CR Example 16 28.6675 250 CR Comparative Example 17 27.6 675 350 CR Comparative Example 1829.6 675 800 CR Comparative Example 19 25.9 675 700 EG ComparativeExample 20 25.0 675 500 CR Comparative Example 21 27.6 675 600 CRComparative Example 22 10.3 675 550 CR Example 23  0.0 675 550 CRExample 24 57.5 675 650 CR Comparative Example 25 36.4 675 650 CRComparative Example 26 28.6 520 750 CR Comparative Example 27 29.6 850400 Al Comparative Example 28 28.6 675  2 CR Comparative Example 29 28.6675 1500  CR Comparative Example 30 27.6 688 600 CR Example 31 27.6 674700 GI Example 32 24.1 675 500 CR Example 33 25.0 672 550 Al Example 3428.6 700 540 CR Example 35 28.6 676 290 GA Example 36 28.6 680 650 GAExample 37 28.6 690 650 CR Example 38 28.6 710 550 CR Example 39 28.6700 600 CR Example 40 28.6 700 600 GA Example 41 28.6 690 600 GA Example42 28.6 690 650 CR Example 43 28.6 690 650 CR Example 44 28.6 680 600 CRExample 45 28.6 690 550 CR Example 46 28.6 710 580 GA Example 47 28.6710 600 GA Comparative Example 48 28.6 680 600 GA Comparative Example 4925.9 702 300 GI Comparative Example 50 29.6 749 200 EG ComparativeExample 51 29.6 714 280 CR Comparative Example 52 28.6 673 360 EGComparative Example 53 21.4 669 370 GI Example 54 28.6 680 400 CRExample 55 28.6 680 300 GA Example 56 27.6 681 320 CR Example 57 28.6668 300 EG Example 58 27.6 675 300 Al Example 59 29.6 685 200 GI Example60 29.6 700 250 GI Example 61 28.6 677 280 GI Example 62 27.6 690 250 EGExample 63 22.2 680 300 Al Example 64 25.0 687 340 GA Example 65 28.6671 600 G1 Example 66 29.6 671 500 Al Example 67 29.6 679 500 CR Example68 29.6 671 350 CR Example 69 28.6 692 400 CR Example 70 25.9 657 200 CRExample 71 28.6 658 180 CR Example Underline: outside range according topresent disclosure *CR: cold-rolled steel sheet (no coating), GI:hot-dip galvanized steel sheet (no galvannealing), GA: galvannealedsteel sheet Al: hot-dip aluminum-coated steel sheet, EG:electrogalvanized steel sheet

TABLE 3 Rolling Volume Average Steel reduction in Area ratio Area ratioArea ratio fraction of grain size Aspect ratio of sample cold rolling ofF of F′ of M RA (μm) crystal grains No. ID (%) (%) (%) (%) (%) F M RA FM RA Balance Remarks 1 A 27.6 64.9 8.0  6.3 13.1 4.9 2.6 2.4 4.3 3.5 4.2BF, P, θ Example 2 B 27.6 55.2 9.2 10.6 19.2 4.3 1.7 1.8 4.5 4.0 4.3 BF,P, θ Example 3 C 28.6 42.5 16.9  13.5 24.6 3.2 1.0 1.1 4.4 4.1 4.3 BF,P, θ Example 4 C 28.6 45.1 8.5 13.5 22.5 4.2 2.1 1.2 4.5 4.1 4.2 BF, P,θ Example 5 C 28.6 42.1 9.2 12.6 23.4 4.1 2.0 1.1 4.8 4.0 4.3 BF, P, θExample 6 C 22.2 42.5 10.2  15.4 22.1 3.2 1.9 1.3 4.5 4.1 4.3 BF, P, θExample 7 C 29.6 43.1 9.8 12.5 20.6 3.0 1.8 2.1 4.4 4.2 4.2 BF, P, θExample 8 C 28.6 42.5 9.1 14.1 25.1 3.5 1.3 2.2 5.1 4.5 4.3 BF, P, θExample 9 C 25.9 41.6 9.1 13.8 22.6 3.6 1.5 2.5 4.8 4.2 4.1 BF, P, θExample 10 C 25.9 38.1 10.1  12.8 22.4 3.8 2.0 1.2 4.5 4.1 4.1 BF, P, θExample 11 C 28.6 50.6 11.2  14.1 23.5 5.0 2.1 1.3 4.5 4.2 4.2 BF, P, θExample 12 C 25.0 32.1 17.2  17.8 24.8 4.5 2.3 1.5 4.7 4.0 4.2 BF, P, θExample 13 C 25.0 46.9 12.1  12.9 25.1 3.9 1.5 1.5 4.8 4.0 4.3 BF, P, θExample 14 C 22.2 41.0 11.2  14.1 22.6 5.0 1.9 1.6 4.7 4.1 4.4 BF, P, θExample 15 C 28.6 42.6 12.5  16.5 23.9 4.1 2.1 1.4 4.8 4.1 4.1 BF, P, θExample 16 C 28.6 55.2 10.2  12.8  7.1 3.9 1.5 1.6 4.4 4.2 4.5 BF, P, θComparative Example 17 C 27.6 60.8 10.8  13.4  7.4 4.1 1.8 1.4 3.5 4.04.2 BF, P, θ Comparative Example 18 C 29.6 55.9 10.3  12.5 18.6 7.8 4.24.1 5.5 4.1 4.5 BF, P, θ Comparative Example 19 C 25.9 59.1 10.4  15.4 6.4 5.4 1.8 1.7 4.4 4.3 4.4 BF, P, θ Comparative Example 20 C 25.0 59.312.1  16.8  6.9 5.2 1.5 1.4 4.6 4.4 4.1 BF, P, θ Comparative Example 21C 27.6 50.2 11.4  14.9  6.2 5.1 1.6 1.5 4.7  4.4. 4.3 BF, P, θComparative Example 22 C 10.3 40.1 9.3 10.4 18.5 4.2 2.1 2.0 5.8 4.8 4.1BF, P, θ Example 23 C  0.0 45.3 9.3 10.4 18.5 4.2 2.1 2.0 8.1 4.2 4.1BF, P, θ Example 24 C 57.5 70.0 10.8   8.0  8.0 6.5 2.7 2.9 1.4 1.4 1.5BF, P, θ Comparative Example 25 C 36.4 65.5 10.8   8.5  8.1 5.1 2.7 2.91.9 1.8 1.9 BF, P, θ Comparative Example 26 C 28.6 55.8 10.4  18.1  6.24.5 2.3 2.1 3.8 4.2 4.3 BF, P, θ Comparative Example 27 C 29.6 53.810.9  17.1  6.5 4.6 2.1 2.4 4.2 4.5 4.6 BF, P, θ Comparative Example 28C 28.6 54.2 11.2  18.5  6.3 4.2 2.3 2.2 3.9 4.0 4.7 BF, P, θ ComparativeExample 29 C 28.6 58.2 1.2  8.4 23.4 5.6 4.1 3.9 4.1 3.8 4.1 BF, P, θComparative Example 30 D 27.6 50.1 9.8 10.4 18.4 4.2 1.8 1.8 4.2 4.2 5.0BF, P, θ Example 31 E 27.6 52.1 10.1  10.6 16.8 4.1 1.5 1.9 4.5 4.4 4.2BF, P, θ Example 32 F 24.1 50.1 9.8 10.9 19.2 3.9 1.9 1.8 4.5 4.7 4.4BF, P, θ Example 33 G 25.0 64.5 8.1  8.5 12.8 5.1 2.7 2.3 4.4 4.8 4.3BF, P, θ Example 34 H 28.6 63.1 8.5  8.8 13.8 4.8 2.5 2.2 4.6 4.6 4.3BF, P, θ Example 35 I 28.6 64.7 8.0  8.8 13.5 4.6 2.4 2.4 4.7 4.5 4.2BF, P, θ Example 36 IA 28.6 64.8 8.5  8.5 12.6 5.1 2.5 1.8 4.5 4.5 4.2BF, P, θ Example 37 IB 28.6 65.0 9.1  8.5 13.5 5.5 2.6 1.5 4.5 4.5 4.3BF, P, θ Example 38 IC 28.6 64.8 8.1  8.9 14.0 5.1 2.4 2.2 4.4 4.6 4.1BF, P, θ Example 39 ID 28.6 55.1 9.1 10.1 18.1 4.9 2.1 1.9 4.2 4.2 4.2BF, P, θ Example 40 IE 28.6 52.0 9.2 10.2 15.6 4.6 2.2 2.0 4.4 4.1 4.3BF, P, θ Example 41 IF 28.6 54.0 9.8 10.1 15.7 4.2 2.3 2.1 4.2 4.6 4.2BF, P, θ Example 42 IG 28.6 53.1 8.7 10.2 16.8 4.5 2.1 2.2 4.5 4.3 4.1BF, P, θ Example 43 IH 28.6 59.1 10.1  10.5 17.2 5.1 2.5 1.9 4.1 4.2 4.4BF, P, θ Example 44 II 28.6 60.1 8.9 10.5 18.1 4.9 2.4 1.5 4.5 4.1 4.1BF, P, θ Example 45 IJ 28.6 58.1 9.5 10.9 15.4 4.6 2.5 1.8 4.7 4.2 4.5BF, P, θ Example 46 IK 28.6 61.2 8.5 11.1 16.9 4.5 2.4 1.5 4.6 4.1 4.3BF, P, θ Example 47 IL 28.6 66.1 8.0 10.5 10.2 5.6 2.0 1.9 4.1 4.1 4.1BF, P, θ Comparative Example 48 IM 28.6 63.5 12.5  14.5  9.0 4.0 2.1 2.14.5 4.5 4.2 BF, P, θ Comparative Example 49 J 25.9 65.0 8.9  3.8  3.67.4 0.6 0.5 4.3 4.4 4.2 BF, P, θ Comparative Example 50 K 29.6 50.418.4  15.4  6.8 5.4 3.9 3.8 4.4 4.1 4.1 BF, P, θ Comparative Example 51L 29.6 60.3 13.2  15.9  6.2 5.8 4.5 3.9 4.4 4.1 4.1 BF, P, θ ComparativeExample 52 M 28.6 50.3 2.2 11.4 13.2 7.1 4.1 4.0 4.2 4.1 3.9 BF, P, θComparative Example 53 N 21.4 55.1 10.4  11.4 18.4 4.2 1.6 1.9 4.3 4.24.1 BF, P, θ Example 54 O 28.6 54.2 10.5   9.8 17.4 4.3 1.7 1.4 4.2 4.34.2 BF, P, θ Example 55 P 28.6 55.7 10.1  10.5 19.5 4.1 1.4 1.5 3.9 4.34.6 BF, P, θ Example 56 Q 27.6 56.1 10.3  10.8 19.6 4.3 1.7 1.6 4.0 4.24.3 BF, P, θ Example 57 R 28.6 65.0 8.2  7.5 12.8 5.1 2.5 2.3 3.9 4.14.5 BF, P, θ Example 58 S 27.6 64.8 8.1  8.2 12.4 4.8 2.4 2.1 4.3 4.24.3 BF, P, θ Example 59 T 29.6 60.9 8.0  7.2 14.2 4.6 2.3 2.1 4.2 4.54.4 BF, P, θ Example 60 U 29.6 63.9 8.1  7.6 13.5 4.5 2.4 2.5 4.4 4.24.5 BF, P, θ Example 61 V 28.6 65.0 8.7  7.5 12.9 4.8 2.1 2.4 4.0 4.14.7 BF, P, θ Example 62 W 27.6 64.9 8.0  6.8 13.2 4.7 2.3 2.1 3.9 3.94.1 BF, P, θ Example 63 X 22.2 50.7 10.4  10.2 18.4 4.1 1.6 1.7 3.7 3.14.2 BF, P, θ Example 64 Y 25.0 62.8 10.5  10.6 15.0 4.5 1.7 1.8 3.9 4.24.4 BF, P, θ Example 65 Z 28.6 55.5 9.8  9.8 20.1 4.2 1.8 1.6 4.2 4.24.3 BF, P, θ Example 66 AA 29.6 50.8 10.6  10.6 19.4 3.9 1.9 1.5 4.1 4.14.2 BF, P, θ Example 67 AB 29.6 52.1 10.1   9.6 18.4 4.1 1.7 1.5 4.2 4.54.6 BF, P, θ Example 68 AC 29.6 52.9 9.9 10.3 18.2 4.2 1.6 1.8 4.4 4.54.7 BF, P, θ Example 69 AD 28.6 51.2 11.1  12.1 19.6 3.9 1.4 1.6 4.3 4.34.1 BF, P, θ Example 70 B 25.9 48.0 10.1  11.2 18.4 4.5 1.9 1.7 4.1 4.24.2 BF, P, θ Example 71 B 28.6 50.5 10.5  10.8 19.1 4.6 1.8 1.8 3.9 4.44.4 BF, P, θ Example Underline: outside range according to presentdisclosure F: polygonal ferrite, F′: non-recrystallized ferrite, BF:bainitic ferrite, RA: retained austenite, M: martensite, P: pearlite, θ:carbide (such as cementite)

TABLE 4 Value obtained by dividing RA remaining volume Average fractionafter Mn content 0.09 × (Mn 0.09 × (Mn 10% tensile Average Average inRA/ content in content in working by Steel Mn content Mn content AverageRA) − 0.026 − RA) − 0.026 + C content RA volume sample in RA in F Mncontent 0.150 0.150 in RA fraction before No. ID (mass %) (mass %) in F(mass %) (mass %) (mass %) working Remarks 1 A 6.89 2.84 2.43 0.4440.744 0.63 0.77 Example 2 B 7.68 3.02 2.54 0.515 0.815 0.71 0.81 Example3 C 8.22 3.08 2.67 0.564 0.864 0.75 0.68 Example 4 C 7.88 2.87 2.750.533 0.833 0.75 0.65 Example 5 C 8.02 2.88 2.78 0.546 0.846 0.75 0.68Example 6 C 7.55 3.01 2.51 0.504 0.804 0.78 0.74 Example 7 C 8.01 3.012.66 0.545 0.845 0.68 0.74 Example 8 C 7.89 3.05 2.59 0.534 0.834 0.740.65 Example 9 C 7.99 2.98 2.68 0.543 0.843 0.78 0.68 Example 10 C 7.752.99 2.59 0.522 0.822 0.75 0.71 Example 11 C 7.98 3.01 2.65 0.542 0.8420.75 0.64 Example 12 C 5.87 2.89 2.03 0.352 0.652 0.62 0.51 Example 13 C7.89 2.89 2.73 0.534 0.834 0.76 0.71 Example 14 C 7.99 3.05 2.62 0.5430.843 0.74 0.72 Example 15 C 7.84 3.01 2.60 0.530 0.830 0.76 0.74Example 16 C 7.45 2.79 2.67 0.495 0.795 0.43 0.25 Comparative Example 17C 7.35 2.89 2.54 0.486 0.786 0.65 0.42 Comparative Example 18 C 6.892.97 2.32 0.444 0.744 0.63 0.51 Comparative Example 19 C 5.41 3.57 1.520.311 0.611 0.50 0.39 Comparative Example 20 C 5.32 3.67 1.45 0.3030.603 0.49 0.52 Comparative Example 21 C 5.26 3.68 1.43 0.297 0.597 0.490.44 Comparative Example 22 C 7.48 2.99 2.50 0.497 0.797 0.69 0.69Example 23 C 7.48 2.99 2.50 0.497 0.797 0.69 0.69 Example 24 C 7.55 2.752.75 0.504 0.804 0.79 0.19 Comparative Example 25 C 7.32 2.75 2.66 0.4830.783 0.79 0.19 Comparative Example 26 C 5.67 3.64 1.56 0.334 0.634 0.290.22 Comparative Example 27 C 5.54 3.75 1.48 0.323 0.623 0.46 0.46Comparative Example 28 C 5.28 3.87 1.36 0.299 0.599 0.49 0.53Comparative Example 29 C 8.26 2.67 3.09 0.567 0.867 0.76 0.39Comparative Example 30 D 7.54 2.89 2.61 0.503 0.803 0.69 0.71 Example 31E 7.32 2.78 2.63 0.483 0.783 0.62 0.75 Example 32 F 7.84 2.97 2.64 0.5300.830 0.72 0.64 Example 33 G 6.95 2.85 2.44 0.450 0.750 0.62 0.69Example 34 H 7.08 2.76 2.57 0.461 0.761 0.63 0.70 Example 35 I 6.85 2.662.58 0.441 0.741 0.61 0.74 Example 36 IA 7.55 3.15 2.40 0.504 0.804 0.550.48 Example 37 IB 7.15 2.58 2.77 0.468 0.768 0.52 0.55 Example 38 IC4.51 1.55 2.91 0.230 0.530 0.51 0.38 Example 39 ID 6.95 3.15 2.21 0.4500.750 0.61 0.68 Example 40 IE 7.10 3.21 2.21 0.463 0.763 0.58 0.70Example 41 IF 6.88 3.01 2.29 0.443 0.743 0.58 0.74 Example 42 IG 7.153.21 2.23 0.468 0.768 0.61 0.74 Example 43 IH 6.99 2.89 2.42 0.453 0.7530.62 0.75 Example 44 II 7.15 2.97 2.41 0.468 0.768 0.65 0.76 Example 45IJ 7.10 2.88 2.47 0.463 0.763 0.58 0.81 Example 46 IK 7.02 2.78 2.530.456 0.756 0.58 0.56 Example 47 IL 6.87 3.10 2.22 0.442 0.742 0.55 0.25Comparative Example 48 IM 6.55 3.15 2.08 0.414 0.714 0.35 0.21Comparative Example 49 J 6.45 2.79 2.31 0.405 0.705 0.38 0.24Comparative Example 50 K 7.29 2.87 2.54 0.480 0.780 0.67 0.52Comparative Example 51 L 3.40 2.08 1.63 0.130 0.430 0.30 0.43Comparative Example 52 M 7.28 2.89 2.52 0.479 0.779 0.65 0.46Comparative Example 53 N 7.59 2.98 2.55 0.507 0.807 0.70 0.62 Example 54O 7.64 2.85 2.68 0.512 0.812 0.65 0.68 Example 55 P 7.49 2.89 2.59 0.4980.798 0.69 0.74 Example 56 Q 7.39 2.98 2.48 0.489 0.789 0.63 0.84Example 57 R 6.58 2.75 2.39 0.416 0.716 0.59 0.86 Example 58 S 6.87 2.812.44 0.442 0.742 0.58 0.81 Example 59 T 6.89 2.74 2.51 0.444 0.744 0.630.75 Example 60 U 6.99 2.65 2.64 0.453 0.753 0.61 0.78 Example 61 V 6.482.71 2.39 0.407 0.707 0.55 0.74 Example 62 W 6.78 2.68 2.53 0.434 0.7340.59 0.76 Example 63 X 7.85 2.89 2.72 0.531 0.831 0.69 0.79 Example 64 Y7.46 2.79 2.67 0.495 0.795 0.66 0.83 Example 65 Z 7.59 2.68 2.83 0.5070.807 0.68 0.74 Example 66 AA 7.36 2.89 2.55 0.486 0.786 0.63 0.78Example 67 AB 7.56 2.78 2.72 0.504 0.804 0.67 0.77 Example 68 AC 7.542.79 2.70 0.503 0.803 0.64 0.68 Example 69 AD 7.94 2.91 2.73 0.539 0.8390.70 0.72 Example 70 B 7.24 3.24 2.23 0.476 0.776 0.45 0.29 Example 71 B7.18 3.28 2.19 0.470 0.770 0.46 0.28 Example Underline: outside rangeaccording to present disclosure F: polygonal ferrite, F′:non-recrystallized ferrite, BF: bainitic ferrite, RA: retainedaustenite, M: martensite, P: pearlite, θ: carbide (such as cementite)

TABLE 5 Steel Sheet Sheet passage Sheet passage Surface sample thicknessability during ability during characteristics of No. ID (mm) hot rollingcold rolling final-annealed sheet Productivity 1 A 2.1 Good Good GoodGood 2 B 2.1 Good Good Good Good 3 C 2.0 Good Good Good Good 4 C 2.0Good Good Good Good 5 C 2.0 Good Good Good Good 6 C 2.1 Good Good GoodGood 7 C 1.9 Good Good Good Good 8 C 2.0 Good Good Good Good 9 C 2.0Good Good Good Good 10 C 2.0 Good Good Good Good 11 C 2.0 Good Good GoodGood 12 C 2.1 Good Good Good Good 13 C 2.1 Good Good Good Good 14 C 2.1Good Good Good Good 15 C 2.0 Good Good Good Good 16 C 2.0 Poor Poor PoorPoor 17 C 2.1 Good Poor Poor Poor 18 C 1.9 Good Good Good Good 19 C 2.0Good Poor Good Good 20 C 2.1 Good Good Good Good 21 C 2.1 Good Poor GoodGood 22 C 2.6 Good Good Good Good 23 C 2.8 Good Good Good Good 24 C 1.7Good Good Good Good 25 C 2.1 Good Good Good Good 26 C 2.0 Good Good GoodGood 27 C 1.9 Good Good Good Poor 28 C 2.0 Good Good Good Good 29 C 2.0Good Good Good Good 30 D 2.1 Good Good Good Good 31 E 2.1 Good Good GoodGood 32 F 2.2 Good Good Good Good 33 G 2.1 Good Good Good Good 34 H 2.0Good Good Good Good 35 I 2.0 Good Good Good Good 36 IA 2.0 Good GoodGood Good 37 IB 2.0 Good Good Good Good 38 IC 2.0 Good Good Good Good 39ID 2.0 Good Good Good Good 40 IE 2.0 Good Good Good Good 41 IF 2.0 GoodGood Good Good 42 IG 2.0 Good Good Good Good 43 IH 2.0 Good Good GoodGood 44 II 2.0 Good Good Good Good 45 IJ 2.0 Good Good Good Good 46 IK2.0 Good Good Good Good 47 IL 2.0 Good Good Good Good 48 IM 2.0 GoodGood Good Good 49 J 2.0 Good Good Good Good 50 K 1.9 Good Good Poor Good51 L 1.9 Good Good Good Good 52 M 2.0 Good Good Good Good 53 N 2.2 GoodGood Good Good 54 O 2.0 Good Good Good Good 55 P 2.0 Good Good Good Good56 Q 2.1 Good Good Good Good 57 R 2.0 Good Good Good Good 58 S 2.1 GoodGood Good Good 59 T 1.9 Good Good Good Good 60 U 1.9 Good Good Good Good61 V 2.0 Good Good Good Good 62 W 2.1 Good Good Good Good 63 X 2.1 GoodGood Good Good 64 Y 2.1 Good Good Good Good 65 Z 2.0 Good Good Good Good66 AA 1.9 Good Good Good Good 67 AB 1.9 Good Good Good Good 68 AC 1.9Good Good Good Good 69 AD 2.0 Good Good Good Good 70 B 2.0 Good GoodGood Good 71 B 2.0 Good Good Good Good YP YR TS EL TS × EL λ No. (MPa)(%) (MPa) (%) (MPa · %) R/t (%) Remarks 1 515 79.2 650 38.2 24830 0.1 61Example 2 691 85.1 812 35.1 28501 0.2 52 Example 3 996 97.8 1018 31.431965 0.5 40 Example 4 990 97.9 1011 32.0 32352 0.5 49 Example 5 95096.4 985 31.5 31028 0.5 52 Example 6 950 96.5 984 31.4 30898 0.5 51Example 7 960 95.8 1002 35.1 35170 0.5 52 Example 8 950 96.3 987 32.632176 0.5 51 Example 9 940 94.9 990 35.1 34749 0.5 49 Example 10 95094.1 1010 33.1 33431 0.5 52 Example 11 950 94.8 1002 31.2 31262 0.5 55Example 12 980 97.5 1005 31.8 31959 0.5 56 Example 13 900 91.5 984 31.530996 0.5 52 Example 14 910 91.9 990 31.4 31086 0.5 51 Example 15 95095.0 1000 32.1 32100 0.5 51 Example 16 695 82.7 840 21.9 18396 1.5 25Comparative Example 17 689 83.5 825 22.4 18480 1.4 30 ComparativeExample 18 501 91.3 549 28.4 15592 1.1 28 Comparative Example 19 69982.8 844 22.5 18990 0.5 44 Comparative Example 20 691 80.3 861 23.520234 0.5 43 Comparative Example 21 663 80.9 820 23.7 19434 0.5 42Comparative Example 22 653 76.8 850 35.9 30515 0.6 56 Example 23 64976.7 846 35.9 30371 0.5 56 Example 24 659 79.4 830 21.6 17928 1.9 22Comparative Example 25 654 79.7 821 21.6 17734 1.5 22 ComparativeExample 26 697 80.1 870 21.5 18705 0.5 31 Comparative Example 27 70582.0 860 20.6 17716 0.5 41 Comparative Example 28 694 79.0 878 20.918350 0.5 40 Comparative Example 29 488 58.8 830 36.2 30046 1.3 25Comparative Example 30 693 84.0 825 34.9 28793 0.2 41 Example 31 66177.6 852 32.8 27946 0.4 45 Example 32 702 82.8 848 35.1 29765 0.2 46Example 33 518 79.7 650 37.8 24570 0.1 59 Example 34 512 81.4 629 38.624279 0.1 65 Example 35 519 82.6 628 38.9 24429 0.1 65 Example 36 51085.7 595 41.0 24395 0.5 60 Example 37 610 93.8 650 38.0 24700 0.5 62Example 38 550 92.0 598 42.0 25116 0.5 58 Example 39 600 91.9 653 38.024814 0.5 55 Example 40 580 93.1 623 40.2 25045 0.5 56 Example 41 56088.9 630 39.1 24633 0.5 60 Example 42 550 88.3 623 38.6 24048 0.5 55Example 43 510 85.4 597 42.5 25373 0.5 52 Example 44 560 82.4 680 35.824344 0.5 55 Example 45 560 86.2 650 39.7 25805 0.5 50 Example 46 65082.5 788 32.1 25295 0.5 56 Example 47 660 84.0 786 30.1 23659 0.5 55Comparative Example 48 660 83.7 789 21.4 16885 0.5 53 ComparativeExample 49 330 60.6 545 31.9 17386 0.1 63 Comparative Example 50 90575.7 1195 15.8 18881 1.3 12 Comparative Example 51 631 76.0 830 20.817264 0.8 37 Comparative Example 52 499 59.1 845 28.9 24421 1.0 15Comparative Example 53 674 81.2 830 36.4 30212 0.2 40 Example 54 66177.7 851 35.8 30466 0.3 45 Example 55 709 87.5 810 35.9 29079 0.4 51Example 56 710 89.0 798 36.4 29047 0.2 48 Example 57 505 80.8 625 39.424625 0.1 53 Example 58 531 83.0 640 39.5 25280 0.1 58 Example 59 50183.4 601 41.2 24761 0.1 55 Example 60 534 78.1 684 37.5 25650 0.1 65Example 61 550 83.5 659 36.9 24317 0.1 55 Example 62 545 82.1 664 36.724369 0.1 59 Example 63 726 83.5 869 34.8 30241 0.2 48 Example 64 70982.5 859 35.1 30151 0.4 45 Example 65 710 85.5 830 36.4 30212 0.3 50Example 66 678 82.6 821 36.9 30295 0.4 47 Example 67 661 81.1 815 34.828362 0.3 53 Example 68 702 84.9 827 35.2 29110 0.3 44 Example 69 78579.2 991 32.6 32307 0.3 42 Example 70 591 74.6 792 31.5 24948 0.5 40Example 71 598 75.3 794 30.8 24455 0.5 41 Example

From above, it can be seen that the steel sheets according to thedisclosure each exhibited TS of 590 MPa or more and YR of 68% or more,and are thus considered as high-strength steel sheets having excellentformability and high yield ratio and hole expansion formability. Incontrast, the comparative examples are inferior in terms of at least oneof YR, TS, EL, λ, or R/t.

INDUSTRIAL APPLICABILITY

According to the disclosure, it becomes possible to manufacturehigh-strength steel sheets with excellent formability and high yieldratio and hole expansion formability that exhibit TS of 590 MPa or moreand YR of 68% or more and that satisfy the condition of TS * EL≥24,000MPa·%. Steel sheets according to the disclosure are highly beneficial inindustrial terms, because they can improve fuel efficiency when appliedto, for example, automobile structural parts, by a reduction in theweight of automotive bodies.

1-6. (canceled)
 7. A steel sheet comprising: a chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: 2.60% or more and 4.20% or less, P:0.001% or more and 0.100% or less, S: 0.0200% or less, N: 0.0100% orless, and Ti: 0.005% or more and 0.200% or less, and optionally furthercontaining, in mass %, at least one selected from the group consistingof Al: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, with thebalance consisting of Fe and inevitable impurities; and a steelmicrostructure that contains, in area ratio, 20% or more and 65% or lessof polygonal ferrite, 8% or more of non-recrystallized ferrite, and 5%or more and 25% or less of martensite, and that contains, in volumefraction, 8% or more of retained austenite, where an average aspectratio of crystal grains of each of the polygonal ferrite, themartensite, and the retained austenite is 2.0 or more and 15.0 or less,wherein the polygonal ferrite has an average grain size of 6 μm or less,the martensite has an average grain size of 3 μm or less, the retainedaustenite has an average grain size of 3 μm or less, and a valueobtained by dividing a Mn content in the retained austenite in mass % bya Mn content in the polygonal ferrite in mass % equals 2.0 or more. 8.The steel sheet according to claim 7, wherein the retained austenite hasa C content that satisfies the following formula in relation to the Mncontent in the retained austenite:0.09*[Mn]−0.026−0.150≤[C]≤0.09*[Mn]−0.026+0.150 where [C] is the Ccontent in the retained austenite in mass %, and [Mn] is the Mn contentin the retained austenite in mass %.
 9. A coated steel sheet comprising:the steel sheet according to claim 7; and one selected from a hot-dipgalvanized layer, a galvannealed layer, a hot-dip aluminum-coated layer,and an electrogalvanized layer provided on the steel sheet.
 10. A coatedsteel sheet comprising: the steel sheet according to claim 8; and oneselected from a hot-dip galvanized layer, a galvannealed layer, ahot-dip aluminum-coated layer, and an electrogalvanized layer providedon the steel sheet.
 11. A method for manufacturing the steel sheetaccording to claim 7, the method comprising: (i) heating a steel slabhaving a chemical composition containing, in mass %, C: 0.030% or moreand 0.250% or less, Si: 0.01% or more and 3.00% or less, Mn: 2.60% ormore and 4.20% or less, P: 0.001% or more and 0.100% or less, S: 0.0200%or less, N: 0.0100% or less, and Ti: 0.005% or more and 0.200% or less,and optionally further containing, in mass %, at least one selected fromthe group consisting of Al: 0.01% or more and 2.00% or less, Nb: 0.005%or more and 0.200% or less, B: 0.0003% or more and 0.0050% or less, Ni:0.005% or more and 1.000% or less, Cr: 0.005% or more and 1.000% orless, V: 0.005% or more and 0.500% or less, Mo: 0.005% or more and1.000% or less, Cu: 0.005% or more and 1.000% or less, Sn: 0.002% ormore and 0.200% or less, Sb: 0.002% or more and 0.200% or less, Ta:0.001% or more and 0.010% or less, Ca: 0.0005% or more and 0.0050% orless, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or moreand 0.0050% or less, with the balance consisting of Fe and inevitableimpurities; (ii) hot rolling the steel slab with a finisher deliverytemperature of 750° C. or higher and 1000° C. or lower to obtain a steelsheet; (iii) coiling the steel sheet at 300° C. or higher and 750° C. orlower; (iv) then subjecting the steel sheet to pickling to removescales; (v) retaining the steel sheet in a temperature range of [Ac₁transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 S; (vi) optionally cold rollingthe steel sheet at a rolling reduction of less than 30%; and (vii) thenretaining the steel sheet in a temperature range of [Ac₁ transformationtemperature+10° C.] to [Ac₁ transformation temperature+100° C.] for 20 sto 900 s and subsequently cooling the steel sheet.
 12. A method formanufacturing the steel sheet according to claim 8, the methodcomprising: (i) heating a steel slab having a chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: 2.60% or more and 4.20% or less, P:0.001% or more and 0.100% or less, S: 0.0200% or less, N: 0.0100% orless, and Ti: 0.005% or more and 0.200% or less, and optionally furthercontaining, in mass %, at least one selected from the group consistingof Al: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, with thebalance consisting of Fe and inevitable impurities; (ii) hot rolling thesteel slab with a finisher delivery temperature of 750° C. or higher and1000° C. or lower to obtain a steel sheet; (iii) coiling the steel sheetat 300° C. or higher and 750° C. or lower; (iv) then subjecting thesteel sheet to pickling to remove scales; (v) retaining the steel sheetin a temperature range of [Ac₁ transformation temperature+20° C.] to[Ac₁ transformation temperature+120° C.] for 600 s to 21,600 s; (vi)optionally cold rolling the steel sheet at a rolling reduction of lessthan 30%; and (vii) then retaining the steel sheet in a temperaturerange of [Ac₁ transformation temperature+10° C.] to [Ac₁ transformationtemperature+100° C.] for 20 s to 900 s and subsequently cooling thesteel sheet.
 13. The method according to claim 11, wherein a valueobtained by dividing a volume fraction of the retained austenite afterperforming tensile working with an elongation value of 10% by a volumefraction of the retained austenite before the tensile working equals 0.3or more.
 14. The method according to claim 12, wherein a value obtainedby dividing a volume fraction of the retained austenite after performingtensile working with an elongation value of 10% by a volume fraction ofthe retained austenite before the tensile working equals 0.3 or more.15. A method for manufacturing the coated steel sheet according to claim9, comprising: (i) heating a steel slab having a chemical compositioncontaining, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01%or more and 3.00% or less, Mn: 2.60% or more and 4.20% or less, P:0.001% or more and 0.100% or less, S: 0.0200% or less, N: 0.0100% orless, and Ti: 0.005% or more and 0.200% or less, and optionally furthercontaining, in mass %, at least one selected from the group consistingof Al: 0.01% or more and 2.00% or less, Nb: 0.005% or more and 0.200% orless, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or moreand 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0.005% ormore and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb:0.002% or more and 0.200% or less, Ta: 0.001% or more and 0.010% orless, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, with thebalance consisting of Fe and inevitable impurities; (ii) hot rolling thesteel slab with a finisher delivery temperature of 750° C. or higher and1000° C. or lower to obtain a steel sheet; (iii) coiling the steel sheetat 300° C. or higher and 750° C. or lower; (iv) then subjecting thesteel sheet to pickling to remove scales; (v) retaining the steel sheetin a temperature range of [Ac₁ transformation temperature+20° C.] to[Ac₁ transformation temperature+120° C.] for 600 s to 21,600 s; (vi)optionally cold rolling the steel sheet at a rolling reduction of lessthan 30%; (vii) then retaining the steel sheet in a temperature range of[Ac₁ transformation temperature+10° C.] to [Ac₁ transformationtemperature+100° C.] for 20 s to 900 s and subsequently cooling thesteel sheet; and (viii) thereafter, either subjecting the steel sheetafter cooling to one selected from hot-dip galvanizing treatment,hot-dip aluminum coating treatment, and electrogalvanizing treatment, orsubjecting the steel sheet after cooling to hot-dip galvanizingtreatment and then to alloying treatment at 450° C. or higher and 600°C. or lower.
 16. A method for manufacturing the coated steel sheetaccording to claim 10, comprising: (i) heating a steel slab having achemical composition containing, in mass %, C: 0.030% or more and 0.250%or less, Si: 0.01% or more and 3.00% or less, Mn: 2.60% or more and4.20% or less, P: 0.001% or more and 0.100% or less, S: 0.0200% or less,N: 0.0100% or less, and Ti: 0.005% or more and 0.200% or less, andoptionally further containing, in mass %, at least one selected from thegroup consisting of Al: 0.01% or more and 2.00% or less, Nb: 0.005% ormore and 0.200% or less, B: 0.0003% or more and 0.0050% or less, Ni:0.005% or more and 1.000% or less, Cr: 0.005% or more and 1.000% orless, V: 0.005% or more and 0.500% or less, Mo: 0.005% or more and1.000% or less, Cu: 0.005% or more and 1.000% or less, Sn: 0.002% ormore and 0.200% or less, Sb: 0.002% or more and 0.200% or less, Ta:0.001% or more and 0.010% or less, Ca: 0.0005% or more and 0.0050% orless, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or moreand 0.0050% or less, with the balance consisting of Fe and inevitableimpurities; (ii) hot rolling the steel slab with a finisher deliverytemperature of 750° C. or higher and 1000° C. or lower to obtain a steelsheet; (iii) coiling the steel sheet at 300° C. or higher and 750° C. orlower; (iv) then subjecting the steel sheet to pickling to removescales; (v) retaining the steel sheet in a temperature range of [Ac₁transformation temperature+20° C.] to [Ac₁ transformationtemperature+120° C.] for 600 s to 21,600 s; (vi) optionally cold rollingthe steel sheet at a rolling reduction of less than 30%; (vii) thenretaining the steel sheet in a temperature range of [Ac₁ transformationtemperature+10° C.] to [Ac₁ transformation temperature+100° C.] for 20 sto 900 s and subsequently cooling the steel sheet; and (viii)thereafter, either subjecting the steel sheet after cooling to oneselected from hot-dip galvanizing treatment, hot-dip aluminum coatingtreatment, and electrogalvanizing treatment, or subjecting the steelsheet after cooling to hot-dip galvanizing treatment and then toalloying treatment at 450° C. or higher and 600° C. or lower.